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Reconstruction of the Medial Patellofemoral Ligament Using a Synthetic Graft With Arthroscopic Control of Patellofemoral Congruence
Reconstruction of the Medial Patellofemoral Ligament Using a Synthetic Graft With Arthroscopic Control of Patellofemoral Congruence Jun Suganuma, M.D., Ryuta Mochizuki, M.D., Tadashi Sugiki, M.D., Yutaka Inoue, M.D., Kazuya Kitamura, M.D., Seiji Akutsu, M.D., and Hiroyuki Ono, M.D.
Purpose: To compare the results of reconstruction of the medial patellofemoral ligament (MPFL) using a synthetic graft (Poly-Tape) between knee joints in which the patella was reduced to the strict center and those in which it was slightly lateral to the center of the trochlea to determine whether patellar position within this range affects the results. Methods: Forty-six knee joints in 46 patients were examined retrospectively with a minimum follow-up of 2 years. The position of the patellar central ridge in the trochlea on arthroscopy immediately after reconstruction of the MPFL was measured. The joints were classified into group 1 (6 male and 12 female patients), in which the patella was reduced to the strict center of the trochlea, and group 2 (10 male and 18 female patients), in which the patella was reduced slightly lateral to the center. The mean age was 20.7 years in group 1 and 20.3 years in group 2. Knee joints were assessed using the Kujala score and the International Knee Documentation Committee (IKDC) subjective evaluation score. Results: The mean Kujala and IKDC scores improved significantly in both groups after surgery (both P < .001). There was no significant difference between the groups for any assessment before surgery or in the Kujala score after surgery (P ¼ .075). However, the IKDC score after surgery was significantly better in group 2 (91.3 9.1) than in group 1 (82.8 13.1) (P ¼ .012). Conclusions: When recurrent dislocation of the patella was treated with MPFL reconstruction using a synthetic graft, subjective evaluations were better in knee joints in which the patella was repositioned slightly lateral to the center of the trochlea than in those in which the patella was reduced to the strict center, although there was no significant difference in knee function between them. Level of Evidence: Level III, retrospective comparative study.
M
any studies have reported that reconstruction of the medial patellofemoral ligament (MPFL) for recurrent patellar dislocation provides significant improvements in knee function1-3; however, a variety of postsurgical complications have also been reported.4-7 Most complications have been attributed to incorrect positioning of anchoring sites of the reconstructed MPFL, potentially resulting in patellar maltracking during knee motion, restricted range of motion (ROM), and graft failure. Therefore, many researchers have shed light on determining the anatomic insertion8-10 From the Department of Orthopaedic Surgery, Hiratsuka City Hospital (J.S., R.M., T.S., Y.I., K.K.), Hiratsuka; and Department of Orthopaedic Surgery, Haga Red Cross Hospital (S.A., H.O.), Moka, Japan. The authors report that they have no conflicts of interest in the authorship and publication of this article. Received August 26, 2015; accepted February 5, 2016. Address correspondence to Jun Suganuma, M.D., Department of Orthopaedic Surgery, Hiratsuka City Hospital, 1-19-1 Minamihara, Hiratsuka, Kanagawa 254-0065, Japan. E-mail: [email protected] Ó 2016 by the Arthroscopy Association of North America 0749-8063/15819/$36.00 http://dx.doi.org/10.1016/j.arthro.2016.02.004
and isometric point11,12 of the MPFL on the medial femoral condyle and the patella. Graft tensioning is reported to be another important factor in keeping the patella centered in the trochlear groove without overloading the patellofemoral joint.6 A clinical report showed that anterior knee pain and decreased ROM occurred when the graft was over-tightened.13 A computational analysis showed that the compressive force applied to the medial cartilage increased significantly at 30 to 40 of flexion of the knee joint when a graft was 3 mm shorter than the intact MPFL.14 Cadaveric studies showed that 2 N of tension applied to an MPFL reconstruction stabilized the patella without increasing medial patellofemoral contact pressure15 whereas 10 N of graft tension restored normal patellar tilt, lateral shift, and rotation.16 However, these studies were conducted for knees without apparent patellofemoral malalignment, and thus the study results may not be applied to knees with patellar instability. When a malpositioned femoral attachment site and over-tightened graft were combined, the compressive force applied to the medial cartilage increased
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drastically at a low flexion angle14 and medial subluxation occurred.17 A cadaveric study also showed that graft over-tensioning or femoral tunnels positioned too proximally or distally caused significantly elevated medial joint contact pressure and increased medial patellar tilting.18 These studies showed that both the femoral attachment site and graft tensioning are crucial factors related to reconstructing the MPFL without complications. However, when the patella is reduced to the strict center of the trochlea, a small technical error in femoral anchoring sites or graft tensioning, which might be induced by morphologic variations in the anteroposterior diameter or posterior cortex curvature of the distal femoral metaphysis,10 could result in medial patellofemoral articular overload or a disturbance of deep flexion of the knee joint. Therefore, precautionary measures including slight loosening of the graft would be needed to reduce the possibility of complications. Clinical studies proposed that, while the graft is being secured at 30 of flexion, the patella is pushed slightly laterally to avoid medial over-tensioning.19 The graft is tensioned at 45 of flexion so that the patella can still be lateralized manually 10 mm to avoid overloading.20 Thus, to determine how much lateral shift of the patella should be allowed immediately after surgery, we have controlled patellofemoral congruence at 30 of flexion on arthroscopy using devices that are able to change the length of the reconstructed MPFL while applying external rotation of the knee joint (ER) and electrical stimulation of the quadriceps (ESQ) to keep the patella shifted laterally. The purpose of this study was to compare the results of reconstruction of the MPFL using a synthetic graft between knee joints in which the patella was reduced to the strict center and those in which it was slightly lateral to the center of the trochlea to determine whether patellar position within this range affects the results. We hypothesized that MPFL reconstruction with the patella reduced slightly lateral to the center of the trochlea would have better Kujala score21 and International Knee Documentation Committee (IKDC) subjective evaluation score22 results than with the patella reduced to the strict center.
Methods Institutional review board approval was obtained for this study. To be included in this retrospective study, the following criteria had to be met: (1) patients underwent MPFL reconstruction with a 20-mm 500-mm Poly-Tape (PT20; Neoligaments, Leeds, England) under general anesthesia without muscle relaxants in our hospitals between January 2002 and June 2013; (2) videos of arthroscopy that recorded the patellofemoral joint at 30 of flexion through a proximal superomedial portal23 were available; and (3) the position of the
patellar central ridge (PPCR) with ER and ESQ measured immediately after the MPFL reconstruction was between 0% and 10% of the lateral facet of the trochlea on still video frames (Fig 1). The measurement of PPCR was conducted following the original method.24 We excluded the following from this study: knee joints with previous surgical treatment for patellar instability; habitual or permanent dislocation of the patella; patella alta that needed additional distal advancement of a tibial tubercle; anteroposterior or varus-valgus instability; contracture; scar tissue around the patella; osteoarthritis of grade 2 or higher on the Kellgren-Lawrence grading
Fig 1. Arthroscopic view of a right patellofemoral joint at 30 of flexion using a 45 arthroscope through a proximal superomedial portal with external rotation of the knee joint and electrical stimulation of the quadriceps immediately after reconstruction of the medial patellofemoral ligament, showing how to measure the position of the patellar central ridge in the trochlear groove on a still video frame after correcting for image distortion.24 The tangent touching the medial (M) and lateral (L) articular surfaces of both sides of the trochlear groove is assumed to be the x-axis. The point on the trochlear groove that is the farthest from the x-axis is considered the deepest point (D). The intersection of the x-axis and the line that meets the x-axis at a right angle and passes through D is assumed to be 0 on the x-coordinate, and the lateral point of tangency (L) is assumed to be 100. The intersection between the x-axis and the line that is perpendicular to the x-axis and passes the patellar central ridge (C) is designated as P. The position of P is measured as the position of the patellar central ridge on the x-coordinate. A video showing when the electrical stimulation of the quadriceps is conducted is placed in a small window on the arthroscopy display (picture-in-picture). A part of the video is seen in the lower right corner. The part of the video that overlaps with the round arthroscopic image is distorted by the correction for lens distortion that is applied to the arthroscopic image.
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RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
system on radiographic examination; a tear or anomaly of the meniscus on magnetic resonance imaging; or abnormal morphologies of the patella or trochlea on arthroscopy in which the patellar central ridge or trochlear groove could not be recognized, which makes PPCR measurement impossible. Patients The number of knee joints that underwent MPFL reconstruction for recurrent patellar dislocation in our hospitals between January 2002 and June 2013 was 66 knee joints in 65 patients. The number of times patellar dislocation had occurred ranged from 2 to more than 30, mostly from 3 to 5 times. Of these knee joints, 7 were excluded from this study because distal advancement of a tibial tubercle osteotomy for patella alta was performed simultaneously, 4 were excluded because tendons were used as a graft for the reconstruction because of open physes, 3 were excluded because muscle relaxants were used during anesthesia, 3 were excluded because of osteoarthritic changes on radiographic examination, 2 were excluded because the patellar central ridge or trochlear groove was not recognized because of deformity of the bone tissue, and one was excluded because of previous surgery for patellar instability. PPCR with ER and ESQ at 30 of flexion of the knee joint immediately after MPFL reconstruction was between 0% and 10% in all of the remaining 46 knee joints in 46 patients, and all of them were included in this study. They were classified into 2 groups by PPCR. Group 1 consisted of joints with the patella reduced to the strict center of the trochlea; the PPCR was 0%, which means that the space between the medial articular surfaces of the patella and the trochlea is not recognized on arthroscopy unless there is a large osteochondral fracture on the medial facet of the patella. This group included 18 patients (6 male and 12 female patients), ranging in age from 14 to 31 years (mean, 20.7 years). The follow-up period ranged from 2.1 to 7.8 years (mean, 4.3 years). Lateral retinacular release was conducted simultaneously in 2 patients. Group 2 included joints with the patella reduced slightly lateral to the center of the trochlea; the PPCR was between 1% and 10% (mean, 5.3%), which means that the space between the medial articular surfaces of the patella and the trochlea is clearly recognized on arthroscopy. This group included 28 patients (10 male and 18 female patients), ranging in age from 14 to 34 years (mean, 20.3 years). The follow-up period varied from 2.1 to 8.0 years (mean, 4.0 years). Lateral retinacular release was conducted in 2 patients. The distribution of the knee joints by group and gender, the mean age in the subgroups classified by group and gender, and the mean follow-up period of each group are shown in Table 1.
Table 1. Patient Data No. of knee joints Male Female Total Age, yr Male Female Total Follow-up period, yr
Group 1
Group 2
P Value
6 12 18
10 18 28
>.99*
22.3 19.8 20.7 4.3
6.2 6.0 6.0 1.9
19.4 20.8 20.3 4.0
6.0 6.9 6.5 1.6
.37 .70 .84 .65
NOTE. Data are presented as number or mean standard deviation. *Fisher exact test.
Surgical Technique All operations were performed by the same surgeon (J.S.). Before surgery, all knee joints were examined clinically, and arthroscopy was performed through anterolateral and anteromedial portals to rule out further intra-articular lesions in the tibiofemoral joint. Chondral lesions and the congruence of the patellofemoral joint were then evaluated through a proximal superomedial portal.23 The medial border of the patella was exposed with a length of about 2 cm. A PT20 was passed through a 3.2-mm-diameter bone tunnel that was made transversely through the patella after spotting the anchoring site of the MPFL on the patella under fluoroscopic control. The lateral end of the PT20 was passed over the anterior surface of the patella, deep to the anterior retinaculum, so as to congregate both ends of the PT20. The femoral insertion site of the MPFL (2 mm posterior to the posterior femoral cortex extension line and 2 mm distal to the posterior femoral condyle) was identified on the true lateral view of the femoral condyles under fluoroscopic control. A Kirschner wire with a diameter of 1.2 mm was inserted into the insertion site through a longitudinal posteromedial incision of about 5 cm. The graft tensioner attached to the tensioner guide was introduced through the incision, with the K-wire inserted into the guide tunnel of the tensioner guide and fixed to the femur, with 2 K-wires with diameters of 2.0 mm (Fig 2). The tensioner guide was then disassembled after removal of the K-wire (1.2 mm). Both ends of the PT20 were passed between the second and third layers on the medial side of the knee, over the medial femoral epicondyle, and between the bifurcated tips of the graft tensioner. They were then secured to the graft gripper (Fig 2), which was mounted on the graft tensioner at this time. Both ends of the PT20 were pulled by sliding the graft gripper on the graft tensioner so that the patella was reduced to the planned position. The staple guide was mounted on the graft tensioner, and the graft pusher was introduced through the slit of the staple guide (Fig 2) so that the graft pusher pressed the PT20 to the femur, simulating the final fixation of the PT20 with staples (Fig 3). The following verification
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surgery using continuous passive motion for at least 12 hours a day for at least 7 days with an initial range of 90 , which was increased by 5 a day. Partial weightbearing gait, active ROM exercise without restriction of the flexion angle, and quadriceps exercise were initiated 2 days after surgery, and full weight-bearing gait without a brace was started after 5 days when straight-leg raising was possible. Sports activity was permitted at 3 months.
Fig 2. Devices used for reconstruction of the medial patellofemoral ligament. The graft tensioner (GT), whose tip is bifurcated, is fixed to the femur using 2 K-wires with a diameter of 2.0 mm. The graft gripper (GG) is mounted to the GT to grasp and pull the graft. The K-wire with a diameter of 1.2 mm is introduced into the guide tunnel of the tensioner guide (TG). The graft pusher (GP) is introduced into the slit of the staple guide (SG) and fixed to the SG using the screw at the end of the GP. The staple tamper (ST) is used to tamp spike staples. The wing nut at the tail of the GT is used to fix the TG or the SG to the GT.
was conducted each time the length of the MPFL was adjusted: (1) the knee joint could be extended completely by ESQ without the J-sign if there had been one; (2) the knee joint could be flexed more than 150 smoothly; and (3) there was no patellar maltracking during motion of the knee joint with neutral, external, or internal rotation. If there was a problem, the length of the reconstructed MPFL was adjusted. The location of the patella was measured arthroscopically at 30 of flexion to verify that the patella was reduced to the expected position with ER and ESQ and that the patella was not subluxated medially with neutral rotation of the knee joint. If not, the length of the reconstructed MPFL was adjusted. Then, after the graft pusher was removed from the slit of the staple guide, a spike staple was inserted into the slit and driven into the femur using the staple tamper (Fig 2). The graft tensioner with the staple guide was removed en bloc, and the PT20 was folded back at the staple and fixed doubly using another spike staple. After recording of the patellofemoral congruence on arthroscopy at 30 of flexion with ER and ESQ, the superfluous part of the PT20 was excised. Post-treatment Protocol Identical postoperative protocols were used in all patients. ROM exercise was initiated on the day of
Assessments In all patients, the Kujala score,21 IKDC score,22 Tegner activity score, apprehension sign, J-sign, and radiographic findings (Kellgren-Lawrence grading system) were recorded before and at least 2 years after surgery. The congruence angle25 was measured 3 times (before surgery, 3 months after surgery, and 2 years after surgery) by the same orthopaedist (J.S.) on radiographs taken using the method of Laurin et al.26 but with the knee joint flexed 30 (Fig 4). On arthroscopy, the degree of degenerative change based on the Outerbridge grade for patellar cartilage and osteochondral fractures of the patella was recorded at the time of surgery. The arthroscopic patellofemoral congruence at 30 of flexion with ER and ESQ was measured as PPCR before and immediately after MPFL reconstruction
Fig 3. Surgical techniques. Both ends of a 20-mm 500-mm Poly-Tape (PT20), which is drawn through the patellar bone tunnel, are pulled by sliding the graft gripper (black arrow) on the graft tensioner so that the patella is reduced to the planned position. Then, the staple guide is mounted on the graft tensioner, and the graft pusher is introduced through the slit of the staple guide so that the graft pusher presses the PT20 to the femur (white arrow), simulating the final fixation of the PT20 with staples.
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RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
Fig 4. Radiograph showing how to measure the congruence angle. The angle created by the highest point of the medial femoral condyle (M), the deepest point of the trochlear groove (D), and the highest point of the lateral femoral condyle (L) is the sulcus angle. L1 is the line bisecting the sulcus angle. L2 is the line connecting the central ridge of the patella (C) and D. The angle created by L1 and L2 is the congruence angle. The congruence angle is positive when L2 is located lateral to L1 and negative when L2 is located medial to L1.
using still video frames by the operator (J.S.). All patients were asked about the following at the follow-up examination held at our outpatient clinics at least every 6 months: (1) how long it took to regain the ability to sit squarely on the heels after surgery, when the patient was able to sit squarely; (2) whether the patients felt tightness in the knee joint during activities; (3) whether there was crepitus in the patellofemoral joint during knee motion between 0 and 90 of flexion; (4) whether recurrence of instability of the patella had occurred; and (5) whether complications occurred. Statistical Analysis All statistical analyses were performed with IBM SPSS Statistics (version 20.0; IBM, Armonk, NY) with a significance level of .05. Intraobserver and interobserver reliabilities with respect to the PPCR with ER and ESQ immediately after the MPFL reconstruction were estimated as follows: For intraobserver reliability, the PPCRs of all knee joints were measured on still video frames twice by the same examiner, with a minimum interval of 1 month between measurements, and the intraclass correlation coefficients of group 1, group 2, and all patients were obtained. For interobserver reliability, 2 examiners measured the PPCRs of all knee joints on still video frames, and the intraclass correlation coefficients of group 1, group 2, and all patients were obtained.
The Fisher exact test was used for statistical analysis of the distribution of the knee joints by gender between the 2 groups, and the unpaired t test was used for statistical analysis of the mean age and follow-up period between the 2 groups. When the effects of surgery in each group were studied, the significance of differences in the mean values of the Kujala score, IKDC score, and PPCR between before and after surgery was determined using the paired t test, and that of the congruence angle among measurements was determined using repeatedmeasures analysis of variance. As for the Tegner activity score, the Wilcoxon signed rank test was used, and the McNemar test was used for the presence of the apprehension sign and the J-sign. The significance of differences between the 2 groups in the mean values of the Kujala score, IKDC score, time needed to sit squarely, congruence angle, and PPCR was determined with the unpaired t test (including the Levene test); that in the results for degenerative change of the patellar cartilage, which was categorized into 2 grades (Outerbridge I and II and Outerbridge III and IV), and the Tegner activity score was determined with the Mann-Whitney U test; and that in the occurrences of the J-sign, osteochondral fracture of the patella, tightness, and crepitus was determined using the Fisher exact test.
Results The intraclass correlation coefficients of group 2 and of all patients for intraobserver reliability analysis of the PPCR were 0.77 and 0.93, respectively. Those for interobserver reliability analysis of the PPCR were 0.64 and 0.88, respectively. The intraclass correlation coefficients of group 1 for intraobserver and interobserver reliability analyses of PPCR could not be calculated because each PPCR was always measured to be 0%. There were no significant differences in the distribution of knee joints by gender, mean age, and mean follow-up period between the 2 groups (Table 1). However, there was a discrepancy in the number of patients included in groups 1 and 2. The patients who underwent MPFL reconstruction after January 2002 were allocated alternately to the 2 groups in each gender before surgery. However, there were 2 patients in each gender who had been allocated to group 1 before surgery but ended up belonging to group 2 because the PPCR turned out to not be 0%, owing to correction of patellar maltracking during surgery. Furthermore, the discrepancy eventually increased after the inclusion and exclusion criteria of this study were defined. The Kujala scores, IKDC scores, Tegner activity scores, congruence angles, and PPCRs before and after surgery and the time needed to sit squarely, rates of tightness and crepitus in the knee joint after surgery, and rate of osteochondral fracture of the patella in each group are presented in Table 2.
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Table 2. Results of Assessments of MPFL Reconstruction Group 1 (n ¼ 18) Clinical findings Kujala score Before surgery 2 yr after surgery IKDC score Before surgery 2 yr after surgery Tegner activity score Before surgery 2 yr after surgery Time needed to sit squarely, mo Tightness in knee joint Crepitus in PFJ Radiographic findings Congruence angle, Before surgery 3 mo after surgery 2 yr after surgery Arthroscopic findings Osteochondral fracture of patella PPCR with ER and ESQ, % BR AR
Group 2 (n ¼ 28)
P Value
1 b (a ¼ .05)
75.2 13.4* 96.4 2.3*
73.1 10.9* 97.7 2.3*
.57 .075
.086 .43
69.8 12.4* 82.8 13.1*
69.3 10.7* 91.3 9.1*
.87 .012
.053 .73
4.6 (3 to 7) 4.6 (3 to 7) 4.6 2.7 4 (22%) 5 (28%)
4.6 (3 to 7) 4.6 (3 to 7) 1.8 1.4 0 (0%) 1 (4%)
.96 .78 .001 .019 .028
.050 .057 .99 .77 .68
22.8 20.6* 14.4 5.6 14.0 8.9*
26.8 22.6* 4.1 15.2 3.4 16.2*
.55
.091
.007
.70
6 (33)
10 (36)
>.99
102.1 10.9* 0.0 0.0*
103.4 10.9* 5.3 2.2*
.69 <.001
.053 .067 >.99
NOTE. Data are presented as mean standard deviation, mean (range) or number (percent). The P value represents the comparison between the groups. AR, after reconstruction of MPFL; BR, before reconstruction of MPFL; ER, external rotation of knee joint; ESQ, electrical stimulation of quadriceps muscle; IKDC, International Knee Documentation Committee subjective evaluation; MPFL, medial patellofemoral ligament; PFJ, patellofemoral joint; PPCR, position of patellar central ridge.17 *Significant difference (P < .001) between before surgery and 2 years or more after surgery or between BR and AR.
In group 1, 6 patients had an osteochondral fracture on the medial facet of the patella, 9 had grade I or II degenerative change of the patellar cartilage, and 9 had grade III or IV degenerative change. The apprehension sign was seen in all patients, but it disappeared after surgery (P < .001). The J-sign was apparent in 6 patients, but it disappeared after surgery (P ¼ .031, b ¼ .21). There were significant improvements in the Kujala score, IKDC score, congruence angle, and PPCR after surgery (all P < .001). However, there was no significant difference in the Tegner activity score between before and after surgery (P ¼ .74, b ¼ .94) or the congruence angle between 3 months and 2 years or more after surgery (P > .99). Complications included a case of paresthesia of the infrapatellar region, but this resolved completely in 2 months. In group 2, 10 patients had an osteochondral fracture on the medial facet of the patella, 13 had grade I or II degenerative change of the patellar cartilage, and 15 had grade III or IV degenerative change. All patients had the apprehension sign before surgery, and it disappeared after surgery (P < .001). The J-sign was apparent in 8 patients, and only a trace of the J-sign remained in 1 patient after surgery (P ¼ .016, b ¼ .18). There were significant improvements in the Kujala score, IKDC score, congruence angle, and PPCR after
surgery (all P < .001). However, there was no significant difference in the Tegner activity score between before and after surgery (P ¼ .71, b ¼ .94) or the congruence angle between 3 months and 2 years or more after surgery (P > .99). Complications included 2 cases of paresthesia of the infrapatellar region, but each remitted completely within 2 months. There were no significant differences before surgery in the Kujala score, IKDC score, Tegner activity score, congruence angle, and PPCR between the 2 groups. Moreover, there were no significant differences in the rates of the apprehension sign, J-sign (P ¼ .75, b ¼ .94), degenerative change of the patellar cartilage (P ¼ .82, b ¼ .94), or osteochondral fracture of the patella before surgery. However, the IKDC score and the congruence angle became significantly larger in group 2 than in group 1 (P ¼ .012 and P ¼ .007, respectively) after surgery. The time needed to sit squarely was significantly shorter in group 2 than in group 1 (P ¼ .001), and the rates of tightness and crepitus in the knee joint were significantly higher in group 1 than in group 2 (P ¼ .019 and P ¼ .028, respectively). No significant differences between the 2 groups remained in the Kujala score, Tegner activity score, and J-sign (P > .99, b ¼ .88) after surgery. There were no patients with decreased ROM of the knee joint, progression of osteoarthritis on radiographs, an episode of lateral or medial
RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
instability of the patella, wound infection, fracture of the patella, or graft tear in either group.
Discussion The most important finding of this study is that subjective evaluations are better in knee joints in which the patella is repositioned slightly lateral to the center of the trochlea than in those in which the patella is reduced to the strict center when recurrent dislocation of the patella is treated with MPFL reconstruction using a synthetic graft. In both groups, there was no recurrence of patellar instability, restriction of ROM of the knee joint, or the apprehension sign in all patients after surgery, and the mean values of the congruence angle, Kujala score, and IKDC score improved significantly (all P < .001). There were no significant differences between the 2 groups in the mean values of the Kujala score and IKDC score before surgery and in the Kujala score after surgery. However, the mean value of the IKDC score was significantly higher in group 2 than in group 1 after surgery (P ¼ .012). The reason for the discrepancy between the Kujala score and the IKDC score in the postoperative results seems to be that the latter provided a more precise subjective evaluation, especially among patients who recovered fairly well, whereas the former provided a more objective evaluation, although both scales have sufficient responsiveness.21,27 When the patella was reduced slightly lateral to the center of the trochlea, leaving room for the patella to translate medially, tension of the reconstructed MPFL was not high and medial patellofemoral contact pressure was 0 MPa or close to 0 MPa at 30 of flexion of the knee joint. However, when the patella was reduced to the strict center of the trochlea, patients took a longer time to be able to sit squarely (P < .001); the rate of patients who felt tightness in the knee joint increased (P ¼ .019) during activity related to deep flexion of the knee joint, especially extending the knee joint from deep flexion; and the rate of knee joints that had crepitus in the patellofemoral joint increased (P ¼ .028). Because we used a synthetic graft, which is much stiffer than the native MPFL, there is potential for high tension of the reconstructed MPFL and high contact pressure of the medial patellofemoral joint in this group,14,28 even though medial patellar subluxation or patellar maltracking was not recognized during surgery. Moreover, all of the patients who had crepitus in the patellofemoral joint had an osteochondral fracture on the medial facet of the patella. They recognized the crepitus after surgery, the extent of which showed a slight tendency to decrease with time. Although there was no significant difference in the rate of osteochondral fracture of the patella between the groups (P > .99), the rate of crepitus was significantly higher in group 1 than in group 2 (P ¼ .028), which might show that this discrepancy was attributed to the difference in
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contact pressure on the fracture site between the groups. Therefore, the strict center of the trochlea, which seemed to be the ideal reduced position for the recurrently dislocated patella at first sight, overlapped the critical zone where the morbidity rate of tightness in the knee joint and crepitus in the patellofemoral joint increased, and to obtain a better subjective evaluation, it would be better not to reduce the patella to the strict center of the trochlea using the synthetic graft. Graft tensioning is reported to be an important factor in keeping the patella centered in the trochlear groove without overloading the patellofemoral joint.6 In clinical studies, the risk of undertensioning the graft has been balanced with the risk of overconstraining the patellofemoral joint. Because the patella engages into the trochlea at 30 of flexion, fixation of the graft at 45 of flexion with no tension would avoid overconstraining the graft.5 The reconstructed MPFL should allow 2 to 3 patellar quadrants of translation, without overly constraining the patella.6 In cadaveric studies using semitendinosus tendon grafts, Beck et al.15 reported that tension of 2 N applied to an MPFL reconstruction stabilized the patella without increasing medial patellofemoral contact pressure and that 10 N restricted lateral patellar translation and inappropriately redistributed patellofemoral contact pressures. Stephen et al.,18 using a double-strand gracilis tendon graft, reported that graft tension of 2 N restored intact medial and lateral joint contact pressure and patellar tracking but graft tension of 10 or 30 N caused significant increases in medial pressure and tilt. In contrast, Philippot et al.16 reported that 10 N of graft tension was acceptable to restore normal patellar tilt, lateral shift, and rotation and that more than 20 N induced a major overcorrection. However, because the tightness of the lateral retinaculum and the surface geometry of the trochlea and patella vary among patients, it seems difficult to achieve expected patellofemoral congruence by applying a determined tension to the graft. Therefore, we have tried to obtain the expected patellofemoral congruence under arthroscopic control by changing the length of the reconstructed MPFL at 30 of flexion, where the patella is most vulnerable.29,30 However, if the reconstructed MPFL is slack at the time of acquisition of the expected congruence, recurrence of patellar instability would occur. It has been reported that a recurrently dislocated patella was shifted laterally at the maximum with ER and ESQ performed with the knee joint flexed 30 .24 Under such conditions, therefore, we verified the patellofemoral congruence on arthroscopy before the graft was fixed to the femur. The reason we used ER and ESQ instead of applying a laterally directed external force on the patella to prevent slackness of the graft was that we wanted not only to keep the patella shifted as laterally and physiologically as possible but also to confirm how
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the patella changed its dynamic behavior after MPFL reconstruction. In knee joints without patellar disorders, ESQ developed a tendency to shift the patella to the center of the trochlea with ER.24 In knee joints with patellar instability, on the other hand, ESQ shifted the patella far lateral on the trochlea with ER before MPFL reconstruction and still exerted a laterally directed force on the patella in all patients immediately after the reconstruction, which might suggest that a stiffer or stronger graft may be warranted for the reconstruction.28 There was no significant difference in the congruence angle between 3 months and about 4 years after surgery in this study. However, Kita et al.31 reported that the congruence angle improved immediately after MPFL reconstruction using a double-looped semitendinosus tendon but it returned toward the preoperative value to some extent about 1 year after surgery. Zhao et al.,32 reconstructing the MPFL using a looped semitendinosus tendon, also observed that correction of the congruence angle deteriorated over time, especially in the first 12 postoperative months. Furthermore, Xie et al.,33 comparing the results of MPFL reconstruction using a looped semitendinosus tendon with polyester suture augmentation and that without augmentation, reported that deterioration of the corrected position of the patella was not observed in the former whereas slackness of the reconstructed MPFL increased gradually, especially in the first 24 months, in the latter. They speculated that the reconstructed graft might undergo graft remodeling or stress relaxation.31-33 If a reconstructed MPFL becomes slack, and if the extent of the slackness varies unpredictably by case, it would be difficult to attain consistent results with MPFL reconstruction. One of the reasons the reconstructed MPFL did not become slack in this study was the use of a synthetic graft for the operation. The theoretical strength of the reconstructed MPFL in the present method is as much as 2,100 N, which is about 10 times higher than that of the native MPFL29,34 and about 5 times higher than that of a tendon graft.35 Limitations There are several limitations to this study. First, the number of patients was not sufficient to verify a significant difference in the Kujala score between the 2 groups. When the required sample size was computed under the conditions of a ¼ .05, power of 0.80, d (difference in population means) of 5, s (within-group SD) of 6.4,32,33 and allocation ratio of 1, the required sample size was 27 (total of 54), showing that this study did not have enough patients. However, the mean Kujala scores of both groups were high, and the difference in mean values between the groups was very small. Therefore, even if a significant difference in the Kujala
score had been obtained by increasing the number of patients, the difference would have resulted in less than 2 points. Second, the intraclass correlation coefficients of group 2 for intraobserver and interobserver reliability analyses of the PPCR were 0.77 and 0.64, respectively, which were lower than those previously reported (0.93 and 0.87, respectively).24 The reason for the low reliability of PPCR in this study seems to be that the shape of the patellar central ridge was deformed in some patients and it was difficult in such cases to identify the central ridge accurately using only still video frames. During arthroscopic examination, however, it was possible to identify the central ridge (C in Fig 1) in all cases by pushing the patella from the anterior direction to reduce the patella to the strict center of the trochlea temporarily. Because the deepest point of the trochlea (D in Fig 1) is easy to identify on arthroscopy, the point on the patellar articular surface touching point D is determined to be the patellar central ridge. Third, all operations and measurements of PPCR were performed by the same surgeon, and evaluation of patients was performed at our outpatient clinics mostly by the same surgeon. Therefore, this study was not blinded. Fourth, the femoral anchoring site of the reconstructed MPFL in this study was deviated slightly posteriorly. Because the patella with recurrent dislocation is most vulnerable in the first 30 of flexion,29,30 we thought that, if the reconstructed MPFL became tight around 30 of flexion, it was allowed to be loose at more than 60 of flexion.36,37 With this idea, in our study the femoral anchoring site was deviated posteriorly, which is, however, within the range of the reported area for the anchoring site,10,38-42 to meet the demand of all our patients to sit squarely after surgery. Fifth, whether the J-sign reproduced by ESQ can substitute for the natural J-sign is unknown, because ESQ differs from voluntary contraction of the quadriceps. However, the only knee joints in which the J-sign was detected on clinical examination so far were able to reproduce the J-sign with ESQ during surgery. Sixth, the operative devices were changed for small-sized ones in 2007, which eventually made skin incisions slightly smaller. A small, flat metal electrode to stimulate the quadriceps was also changed to a 23-gauge Cattelan needle (TOP, Tokyo, Japan) in 2007. Because the changed devices and electrode were functionally identical to the previous ones, these changes during this study did not appear to have compromised its results. Seventh, a PT20 was used for MPFL reconstruction, and the theoretical linear stiffness of the graft is 220 N/mm, which is about 7 times stiffer than the native MPFL34 and about 2.5 times stiffer than a tendon graft.35 Therefore, if a tendon graft is used in a study of the same design, different results might be obtained, because a less stiff graft may be more forgiving of over-tensioning that would occur in reduction to the strict center of the trochlea.
RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
Conclusions When recurrent dislocation of the patella was treated with MPFL reconstruction using a synthetic graft, subjective evaluations were better in knee joints in which the patella was repositioned slightly lateral to the center of the trochlea than in those in which the patella was reduced to the strict center, although there was no significant difference in knee function between them.
References 1. Buckens CF, Saris DB. Reconstruction of the medial patellofemoral ligament for treatment of patellofemoral instability: A systematic review. Am J Sports Med 2010;38: 181-188. 2. Howells NR, Barnett AJ, Ahearn N, Ansari A, Eldridge JD. Medial patellofemoral ligament reconstruction: A prospective outcome assessment of a large single centre series. J Bone Joint Surg Br 2012;94:1202-1208. 3. Steiner TM, Torga-Spak R, Teitge RA. Medial patellofemoral ligament reconstruction in patients with lateral patellar instability and trochlear dysplasia. Am J Sports Med 2006;34:1254-1261. 4. Arendt EA, Moeller A, Agel J. Clinical outcomes of medial patellofemoral ligament repair in recurrent (chronic) lateral patella dislocations. Knee Surg Sports Traumatol Arthrosc 2011;19:1909-1914. 5. Parikh SN, Nathan ST, Wall EJ, Eismann EA. Complications of medial patellofemoral ligament reconstruction in young patients. Am J Sports Med 2013;41:1030-1038. 6. Tanaka MJ, Bollier MJ, Andrish JT, Fulkerson JP, Cosgarea AJ. Complications of medial patellofemoral ligament reconstruction: Common technical errors and factors for success: AAOS exhibit selection. J Bone Joint Surg Am 2012;94:e87. 7. Shah JN, Howard JS, Flanigan DC, Brophy RH, Carey JL, Lattermann C. A systematic review of complications and failures associated with medial patellofemoral ligament reconstruction for recurrent patellar dislocation. Am J Sports Med 2012;40:1916-1923. 8. Baldwin JL. The anatomy of the medial patellofemoral ligament. Am J Sports Med 2009;37:2355-2361. 9. LaPrade RF, Engebretsen AH, Ly TV, Johansen S, Wentorf FA, Engebretsen L. The anatomy of the medial part of the knee. J Bone Joint Surg Am 2007;89:2000-2010. 10. Stephen JM, Lumpaopong P, Deehan DJ, Kader D, Amis AA. The medial patellofemoral ligament: Location of femoral attachment and length change patterns resulting from anatomic and nonanatomic attachments. Am J Sports Med 2012;40:1871-1879. 11. Smirk C, Morris H. The anatomy and reconstruction of the medial patellofemoral ligament. Knee 2003;10:221-227. 12. Steensen RN, Dopirak RM, McDonald WG III. The anatomy and isometry of the medial patellofemoral ligament: Implications for reconstruction. Am J Sports Med 2004;32: 1509-1513. 13. Thaunat M, Erasmus PJ. Management of overtight medial patellofemoral ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 2009;17:480-483.
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14. Elias JJ, Cosgarea AJ. Technical errors during medial patellofemoral ligament reconstruction could overload medial patellofemoral cartilage: A computational analysis. Am J Sports Med 2006;34:1478-1485. 15. Beck P, Brown NA, Greis PE, Burks RT. Patellofemoral contact pressures and lateral patellar translation after medial patellofemoral ligament reconstruction. Am J Sports Med 2007;35:1557-1563. 16. Philippot R, Boyer B, Testa R, Farizon F, Moyen B. Study of patellar kinematics after reconstruction of the medial patellofemoral ligament. Clin Biomech 2012;27:22-26. 17. Bollier M, Fulkerson J, Cosgarea A, Tanaka M. Technical failure of medial patellofemoral ligament reconstruction. Arthroscopy 2011;27:1153-1159. 18. Stephen JM, Kaider D, Lumpaopong P, Deehan DJ, Amis AA. The effect of femoral tunnel position and graft tension on patellar contact mechanics and kinematics after medial patellofemoral ligament reconstruction. Am J Sports Med 2014;42:364-372. 19. Lippacher S, Dreyhaupt J, Williams SR, Reichel H, Nelitz M. Reconstruction of the medial patellofemoral ligament: Clinical outcomes and return to sports. Am J Sports Med 2014;42:1661-1668. 20. Christiansen SE, Jacobsen BW, Lund B, Lind M. Reconstruction of the medial patellofemoral ligament with gracilis tendon autograft in transverse patellar drill holes. Arthroscopy 2008;24:82-87. 21. Kujala UM, Jaakkola LH, Koskinen SK, Taimela S, Hurme M, Nelimarkka O. Scoring of patellofemoral disorders. Arthroscopy 1993;9:159-163. 22. Irrgang JJ, Anderson AF, Boland AL, et al. Development and validation of the International Knee Documentation Committee subjective knee form. Am J Sports Med 2001;29:600-613. 23. Schreiber SN. Proximal superomedial portal in arthroscopy of the knee. Arthroscopy 1991;7:246-251. 24. Suganuma J, Mochizuki R, Inoue Y, Kitamura K, Honda A. Arthroscopic evaluation of patellofemoral congruence with rotation of the knee joint and electrical stimulation of the quadriceps. Arthroscopy 2014;30: 214-221. 25. Merchant AC, Mercer RL, Jacobsen RH, Cool CR. Roentgenographic analysis of patellofemoral congruence. J Bone Joint Surg Am 1974;56:1391-1396. 26. Laurin CA, Lévesque HP, Dussault R, Labelle H, Peides JP. The abnormal lateral patellofemoral angle: A diagnostic roentgenographic sign of recurrent patellar subluxation. J Bone Joint Surg Am 1978;60:55-60. 27. Irrgang JJ, Anderson AF, Boland AL, et al. Responsiveness of the International Knee Documentation Committee Subjective Knee Form. Am J Sports Med 2006;34: 1567-1573. 28. Feller JA, Amis AA, Andrish JT, Arendt EA, Erasmus PJ, Powers CM. Surgical biomechanics of the patellofemoral joint. Arthroscopy 2007;23:542-553. 29. Amis AA, Firer P, Mountney J, Senavongse W, Thomas NP. Anatomy and biomechanics of the medial patellofemoral ligament. Knee 2003;10:215-220. 30. Philippot R, Boyer B, Testa R, Farizon F, Moyen B. The role of the medial ligamentous structures on patellar
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J. SUGANUMA ET AL. tracking during knee flexion. Knee Surg Sports Traumatol Arthrosc 2012;20:331-336. Kita K, Horibe S, Toritsuka Y, et al. Effects of medial patellofemoral ligament reconstruction on patellar tracking. Knee Surg Sports Traumatol Arthrosc 2012;20: 829-837. Zhao J, Huangfu X, He Y. The role of medial retinaculum plication versus medial patellofemoral ligament reconstruction in combined procedures for recurrent patellar instability in adults. Am J Sports Med 2012;40: 1355-1364. Xie G, Zhao J, Huangfu X, He Y. Medial patellofemoral ligament reconstruction using semitendinosus tendons: Polyester suture augmentation versus nonaugmentation. Am J Sports Med 2012;40:1365-1374. Herbort M, Hoser C, Domnick C, et al. MPFL reconstruction using a quadriceps tendon graft: Part 1: Biomechanical properties of quadriceps tendon MPFL reconstruction in comparison to the intact MPFL. A human cadaveric study. Knee 2014;21:1169-1174. Lenschow S, Schliemann B, Gestring J, Herbort M, Schulze M, Kösters C. Medial patellofemoral ligament reconstruction: Fixation strength of 5 different techniques for graft fixation at the patella. Arthroscopy 2013;29: 766-773.
36. Farr J, Schepsis AA. Reconstruction of the medial patellofemoral ligament for recurrent patellar instability. J Knee Surg 2006;19:307-316. 37. Bicos J, Fulkerson JP, Amis A. Current concepts review: The medial patellofemoral ligament. Am J Sports Med 2007;35:484-492. 38. Schöttle PB, Schmeling A, Rosenstiel N, Weiler A. Radiographic landmarks for femoral tunnel placement in medial patellofemoral ligament reconstruction. Am J Sports Med 2007;35:801-804. 39. Wijdicks CA, Griffith CJ, LaPrade RF, et al. Radiographic identification of the primary medial knee structures. J Bone Joint Surg Am 2009;91:521-529. 40. Redfern J, Kamath G, Burks R. Anatomical confirmation of the use of radiographic landmarks in medial patellofemoral ligament reconstruction. Am J Sports Med 2010;38:293-297. 41. Tateishi T, Tsuchiya M, Motosugi N, et al. Graft length change and radiographic assessment of femoral drill hole position for medial patellofemoral ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 2011;19:400-407. 42. Barnett AJ, Howells NR, Burston BJ, Ansari A, Clark D, Eldridge JD. Radiographic landmarks for tunnel placement in reconstruction of the medial patellofemoral ligament. Knee Surg Sports Traumatol Arthrosc 2012;20: 2380-2384.
Purpose: To compare the results of reconstruction of the medial patellofemoral ligament (MPFL) using a synthetic graft (Poly-Tape) between knee joints in which the patella was reduced to the strict center and those in which it was slightly lateral to the center of the trochlea to determine whether patellar position within this range affects the results. Methods: Forty-six knee joints in 46 patients were examined retrospectively with a minimum follow-up of 2 years. The position of the patellar central ridge in the trochlea on arthroscopy immediately after reconstruction of the MPFL was measured. The joints were classified into group 1 (6 male and 12 female patients), in which the patella was reduced to the strict center of the trochlea, and group 2 (10 male and 18 female patients), in which the patella was reduced slightly lateral to the center. The mean age was 20.7 years in group 1 and 20.3 years in group 2. Knee joints were assessed using the Kujala score and the International Knee Documentation Committee (IKDC) subjective evaluation score. Results: The mean Kujala and IKDC scores improved significantly in both groups after surgery (both P < .001). There was no significant difference between the groups for any assessment before surgery or in the Kujala score after surgery (P ¼ .075). However, the IKDC score after surgery was significantly better in group 2 (91.3 9.1) than in group 1 (82.8 13.1) (P ¼ .012). Conclusions: When recurrent dislocation of the patella was treated with MPFL reconstruction using a synthetic graft, subjective evaluations were better in knee joints in which the patella was repositioned slightly lateral to the center of the trochlea than in those in which the patella was reduced to the strict center, although there was no significant difference in knee function between them. Level of Evidence: Level III, retrospective comparative study.
M
any studies have reported that reconstruction of the medial patellofemoral ligament (MPFL) for recurrent patellar dislocation provides significant improvements in knee function1-3; however, a variety of postsurgical complications have also been reported.4-7 Most complications have been attributed to incorrect positioning of anchoring sites of the reconstructed MPFL, potentially resulting in patellar maltracking during knee motion, restricted range of motion (ROM), and graft failure. Therefore, many researchers have shed light on determining the anatomic insertion8-10 From the Department of Orthopaedic Surgery, Hiratsuka City Hospital (J.S., R.M., T.S., Y.I., K.K.), Hiratsuka; and Department of Orthopaedic Surgery, Haga Red Cross Hospital (S.A., H.O.), Moka, Japan. The authors report that they have no conflicts of interest in the authorship and publication of this article. Received August 26, 2015; accepted February 5, 2016. Address correspondence to Jun Suganuma, M.D., Department of Orthopaedic Surgery, Hiratsuka City Hospital, 1-19-1 Minamihara, Hiratsuka, Kanagawa 254-0065, Japan. E-mail: [email protected] Ó 2016 by the Arthroscopy Association of North America 0749-8063/15819/$36.00 http://dx.doi.org/10.1016/j.arthro.2016.02.004
and isometric point11,12 of the MPFL on the medial femoral condyle and the patella. Graft tensioning is reported to be another important factor in keeping the patella centered in the trochlear groove without overloading the patellofemoral joint.6 A clinical report showed that anterior knee pain and decreased ROM occurred when the graft was over-tightened.13 A computational analysis showed that the compressive force applied to the medial cartilage increased significantly at 30 to 40 of flexion of the knee joint when a graft was 3 mm shorter than the intact MPFL.14 Cadaveric studies showed that 2 N of tension applied to an MPFL reconstruction stabilized the patella without increasing medial patellofemoral contact pressure15 whereas 10 N of graft tension restored normal patellar tilt, lateral shift, and rotation.16 However, these studies were conducted for knees without apparent patellofemoral malalignment, and thus the study results may not be applied to knees with patellar instability. When a malpositioned femoral attachment site and over-tightened graft were combined, the compressive force applied to the medial cartilage increased
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drastically at a low flexion angle14 and medial subluxation occurred.17 A cadaveric study also showed that graft over-tensioning or femoral tunnels positioned too proximally or distally caused significantly elevated medial joint contact pressure and increased medial patellar tilting.18 These studies showed that both the femoral attachment site and graft tensioning are crucial factors related to reconstructing the MPFL without complications. However, when the patella is reduced to the strict center of the trochlea, a small technical error in femoral anchoring sites or graft tensioning, which might be induced by morphologic variations in the anteroposterior diameter or posterior cortex curvature of the distal femoral metaphysis,10 could result in medial patellofemoral articular overload or a disturbance of deep flexion of the knee joint. Therefore, precautionary measures including slight loosening of the graft would be needed to reduce the possibility of complications. Clinical studies proposed that, while the graft is being secured at 30 of flexion, the patella is pushed slightly laterally to avoid medial over-tensioning.19 The graft is tensioned at 45 of flexion so that the patella can still be lateralized manually 10 mm to avoid overloading.20 Thus, to determine how much lateral shift of the patella should be allowed immediately after surgery, we have controlled patellofemoral congruence at 30 of flexion on arthroscopy using devices that are able to change the length of the reconstructed MPFL while applying external rotation of the knee joint (ER) and electrical stimulation of the quadriceps (ESQ) to keep the patella shifted laterally. The purpose of this study was to compare the results of reconstruction of the MPFL using a synthetic graft between knee joints in which the patella was reduced to the strict center and those in which it was slightly lateral to the center of the trochlea to determine whether patellar position within this range affects the results. We hypothesized that MPFL reconstruction with the patella reduced slightly lateral to the center of the trochlea would have better Kujala score21 and International Knee Documentation Committee (IKDC) subjective evaluation score22 results than with the patella reduced to the strict center.
Methods Institutional review board approval was obtained for this study. To be included in this retrospective study, the following criteria had to be met: (1) patients underwent MPFL reconstruction with a 20-mm 500-mm Poly-Tape (PT20; Neoligaments, Leeds, England) under general anesthesia without muscle relaxants in our hospitals between January 2002 and June 2013; (2) videos of arthroscopy that recorded the patellofemoral joint at 30 of flexion through a proximal superomedial portal23 were available; and (3) the position of the
patellar central ridge (PPCR) with ER and ESQ measured immediately after the MPFL reconstruction was between 0% and 10% of the lateral facet of the trochlea on still video frames (Fig 1). The measurement of PPCR was conducted following the original method.24 We excluded the following from this study: knee joints with previous surgical treatment for patellar instability; habitual or permanent dislocation of the patella; patella alta that needed additional distal advancement of a tibial tubercle; anteroposterior or varus-valgus instability; contracture; scar tissue around the patella; osteoarthritis of grade 2 or higher on the Kellgren-Lawrence grading
Fig 1. Arthroscopic view of a right patellofemoral joint at 30 of flexion using a 45 arthroscope through a proximal superomedial portal with external rotation of the knee joint and electrical stimulation of the quadriceps immediately after reconstruction of the medial patellofemoral ligament, showing how to measure the position of the patellar central ridge in the trochlear groove on a still video frame after correcting for image distortion.24 The tangent touching the medial (M) and lateral (L) articular surfaces of both sides of the trochlear groove is assumed to be the x-axis. The point on the trochlear groove that is the farthest from the x-axis is considered the deepest point (D). The intersection of the x-axis and the line that meets the x-axis at a right angle and passes through D is assumed to be 0 on the x-coordinate, and the lateral point of tangency (L) is assumed to be 100. The intersection between the x-axis and the line that is perpendicular to the x-axis and passes the patellar central ridge (C) is designated as P. The position of P is measured as the position of the patellar central ridge on the x-coordinate. A video showing when the electrical stimulation of the quadriceps is conducted is placed in a small window on the arthroscopy display (picture-in-picture). A part of the video is seen in the lower right corner. The part of the video that overlaps with the round arthroscopic image is distorted by the correction for lens distortion that is applied to the arthroscopic image.
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system on radiographic examination; a tear or anomaly of the meniscus on magnetic resonance imaging; or abnormal morphologies of the patella or trochlea on arthroscopy in which the patellar central ridge or trochlear groove could not be recognized, which makes PPCR measurement impossible. Patients The number of knee joints that underwent MPFL reconstruction for recurrent patellar dislocation in our hospitals between January 2002 and June 2013 was 66 knee joints in 65 patients. The number of times patellar dislocation had occurred ranged from 2 to more than 30, mostly from 3 to 5 times. Of these knee joints, 7 were excluded from this study because distal advancement of a tibial tubercle osteotomy for patella alta was performed simultaneously, 4 were excluded because tendons were used as a graft for the reconstruction because of open physes, 3 were excluded because muscle relaxants were used during anesthesia, 3 were excluded because of osteoarthritic changes on radiographic examination, 2 were excluded because the patellar central ridge or trochlear groove was not recognized because of deformity of the bone tissue, and one was excluded because of previous surgery for patellar instability. PPCR with ER and ESQ at 30 of flexion of the knee joint immediately after MPFL reconstruction was between 0% and 10% in all of the remaining 46 knee joints in 46 patients, and all of them were included in this study. They were classified into 2 groups by PPCR. Group 1 consisted of joints with the patella reduced to the strict center of the trochlea; the PPCR was 0%, which means that the space between the medial articular surfaces of the patella and the trochlea is not recognized on arthroscopy unless there is a large osteochondral fracture on the medial facet of the patella. This group included 18 patients (6 male and 12 female patients), ranging in age from 14 to 31 years (mean, 20.7 years). The follow-up period ranged from 2.1 to 7.8 years (mean, 4.3 years). Lateral retinacular release was conducted simultaneously in 2 patients. Group 2 included joints with the patella reduced slightly lateral to the center of the trochlea; the PPCR was between 1% and 10% (mean, 5.3%), which means that the space between the medial articular surfaces of the patella and the trochlea is clearly recognized on arthroscopy. This group included 28 patients (10 male and 18 female patients), ranging in age from 14 to 34 years (mean, 20.3 years). The follow-up period varied from 2.1 to 8.0 years (mean, 4.0 years). Lateral retinacular release was conducted in 2 patients. The distribution of the knee joints by group and gender, the mean age in the subgroups classified by group and gender, and the mean follow-up period of each group are shown in Table 1.
Table 1. Patient Data No. of knee joints Male Female Total Age, yr Male Female Total Follow-up period, yr
Group 1
Group 2
P Value
6 12 18
10 18 28
>.99*
22.3 19.8 20.7 4.3
6.2 6.0 6.0 1.9
19.4 20.8 20.3 4.0
6.0 6.9 6.5 1.6
.37 .70 .84 .65
NOTE. Data are presented as number or mean standard deviation. *Fisher exact test.
Surgical Technique All operations were performed by the same surgeon (J.S.). Before surgery, all knee joints were examined clinically, and arthroscopy was performed through anterolateral and anteromedial portals to rule out further intra-articular lesions in the tibiofemoral joint. Chondral lesions and the congruence of the patellofemoral joint were then evaluated through a proximal superomedial portal.23 The medial border of the patella was exposed with a length of about 2 cm. A PT20 was passed through a 3.2-mm-diameter bone tunnel that was made transversely through the patella after spotting the anchoring site of the MPFL on the patella under fluoroscopic control. The lateral end of the PT20 was passed over the anterior surface of the patella, deep to the anterior retinaculum, so as to congregate both ends of the PT20. The femoral insertion site of the MPFL (2 mm posterior to the posterior femoral cortex extension line and 2 mm distal to the posterior femoral condyle) was identified on the true lateral view of the femoral condyles under fluoroscopic control. A Kirschner wire with a diameter of 1.2 mm was inserted into the insertion site through a longitudinal posteromedial incision of about 5 cm. The graft tensioner attached to the tensioner guide was introduced through the incision, with the K-wire inserted into the guide tunnel of the tensioner guide and fixed to the femur, with 2 K-wires with diameters of 2.0 mm (Fig 2). The tensioner guide was then disassembled after removal of the K-wire (1.2 mm). Both ends of the PT20 were passed between the second and third layers on the medial side of the knee, over the medial femoral epicondyle, and between the bifurcated tips of the graft tensioner. They were then secured to the graft gripper (Fig 2), which was mounted on the graft tensioner at this time. Both ends of the PT20 were pulled by sliding the graft gripper on the graft tensioner so that the patella was reduced to the planned position. The staple guide was mounted on the graft tensioner, and the graft pusher was introduced through the slit of the staple guide (Fig 2) so that the graft pusher pressed the PT20 to the femur, simulating the final fixation of the PT20 with staples (Fig 3). The following verification
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surgery using continuous passive motion for at least 12 hours a day for at least 7 days with an initial range of 90 , which was increased by 5 a day. Partial weightbearing gait, active ROM exercise without restriction of the flexion angle, and quadriceps exercise were initiated 2 days after surgery, and full weight-bearing gait without a brace was started after 5 days when straight-leg raising was possible. Sports activity was permitted at 3 months.
Fig 2. Devices used for reconstruction of the medial patellofemoral ligament. The graft tensioner (GT), whose tip is bifurcated, is fixed to the femur using 2 K-wires with a diameter of 2.0 mm. The graft gripper (GG) is mounted to the GT to grasp and pull the graft. The K-wire with a diameter of 1.2 mm is introduced into the guide tunnel of the tensioner guide (TG). The graft pusher (GP) is introduced into the slit of the staple guide (SG) and fixed to the SG using the screw at the end of the GP. The staple tamper (ST) is used to tamp spike staples. The wing nut at the tail of the GT is used to fix the TG or the SG to the GT.
was conducted each time the length of the MPFL was adjusted: (1) the knee joint could be extended completely by ESQ without the J-sign if there had been one; (2) the knee joint could be flexed more than 150 smoothly; and (3) there was no patellar maltracking during motion of the knee joint with neutral, external, or internal rotation. If there was a problem, the length of the reconstructed MPFL was adjusted. The location of the patella was measured arthroscopically at 30 of flexion to verify that the patella was reduced to the expected position with ER and ESQ and that the patella was not subluxated medially with neutral rotation of the knee joint. If not, the length of the reconstructed MPFL was adjusted. Then, after the graft pusher was removed from the slit of the staple guide, a spike staple was inserted into the slit and driven into the femur using the staple tamper (Fig 2). The graft tensioner with the staple guide was removed en bloc, and the PT20 was folded back at the staple and fixed doubly using another spike staple. After recording of the patellofemoral congruence on arthroscopy at 30 of flexion with ER and ESQ, the superfluous part of the PT20 was excised. Post-treatment Protocol Identical postoperative protocols were used in all patients. ROM exercise was initiated on the day of
Assessments In all patients, the Kujala score,21 IKDC score,22 Tegner activity score, apprehension sign, J-sign, and radiographic findings (Kellgren-Lawrence grading system) were recorded before and at least 2 years after surgery. The congruence angle25 was measured 3 times (before surgery, 3 months after surgery, and 2 years after surgery) by the same orthopaedist (J.S.) on radiographs taken using the method of Laurin et al.26 but with the knee joint flexed 30 (Fig 4). On arthroscopy, the degree of degenerative change based on the Outerbridge grade for patellar cartilage and osteochondral fractures of the patella was recorded at the time of surgery. The arthroscopic patellofemoral congruence at 30 of flexion with ER and ESQ was measured as PPCR before and immediately after MPFL reconstruction
Fig 3. Surgical techniques. Both ends of a 20-mm 500-mm Poly-Tape (PT20), which is drawn through the patellar bone tunnel, are pulled by sliding the graft gripper (black arrow) on the graft tensioner so that the patella is reduced to the planned position. Then, the staple guide is mounted on the graft tensioner, and the graft pusher is introduced through the slit of the staple guide so that the graft pusher presses the PT20 to the femur (white arrow), simulating the final fixation of the PT20 with staples.
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RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
Fig 4. Radiograph showing how to measure the congruence angle. The angle created by the highest point of the medial femoral condyle (M), the deepest point of the trochlear groove (D), and the highest point of the lateral femoral condyle (L) is the sulcus angle. L1 is the line bisecting the sulcus angle. L2 is the line connecting the central ridge of the patella (C) and D. The angle created by L1 and L2 is the congruence angle. The congruence angle is positive when L2 is located lateral to L1 and negative when L2 is located medial to L1.
using still video frames by the operator (J.S.). All patients were asked about the following at the follow-up examination held at our outpatient clinics at least every 6 months: (1) how long it took to regain the ability to sit squarely on the heels after surgery, when the patient was able to sit squarely; (2) whether the patients felt tightness in the knee joint during activities; (3) whether there was crepitus in the patellofemoral joint during knee motion between 0 and 90 of flexion; (4) whether recurrence of instability of the patella had occurred; and (5) whether complications occurred. Statistical Analysis All statistical analyses were performed with IBM SPSS Statistics (version 20.0; IBM, Armonk, NY) with a significance level of .05. Intraobserver and interobserver reliabilities with respect to the PPCR with ER and ESQ immediately after the MPFL reconstruction were estimated as follows: For intraobserver reliability, the PPCRs of all knee joints were measured on still video frames twice by the same examiner, with a minimum interval of 1 month between measurements, and the intraclass correlation coefficients of group 1, group 2, and all patients were obtained. For interobserver reliability, 2 examiners measured the PPCRs of all knee joints on still video frames, and the intraclass correlation coefficients of group 1, group 2, and all patients were obtained.
The Fisher exact test was used for statistical analysis of the distribution of the knee joints by gender between the 2 groups, and the unpaired t test was used for statistical analysis of the mean age and follow-up period between the 2 groups. When the effects of surgery in each group were studied, the significance of differences in the mean values of the Kujala score, IKDC score, and PPCR between before and after surgery was determined using the paired t test, and that of the congruence angle among measurements was determined using repeatedmeasures analysis of variance. As for the Tegner activity score, the Wilcoxon signed rank test was used, and the McNemar test was used for the presence of the apprehension sign and the J-sign. The significance of differences between the 2 groups in the mean values of the Kujala score, IKDC score, time needed to sit squarely, congruence angle, and PPCR was determined with the unpaired t test (including the Levene test); that in the results for degenerative change of the patellar cartilage, which was categorized into 2 grades (Outerbridge I and II and Outerbridge III and IV), and the Tegner activity score was determined with the Mann-Whitney U test; and that in the occurrences of the J-sign, osteochondral fracture of the patella, tightness, and crepitus was determined using the Fisher exact test.
Results The intraclass correlation coefficients of group 2 and of all patients for intraobserver reliability analysis of the PPCR were 0.77 and 0.93, respectively. Those for interobserver reliability analysis of the PPCR were 0.64 and 0.88, respectively. The intraclass correlation coefficients of group 1 for intraobserver and interobserver reliability analyses of PPCR could not be calculated because each PPCR was always measured to be 0%. There were no significant differences in the distribution of knee joints by gender, mean age, and mean follow-up period between the 2 groups (Table 1). However, there was a discrepancy in the number of patients included in groups 1 and 2. The patients who underwent MPFL reconstruction after January 2002 were allocated alternately to the 2 groups in each gender before surgery. However, there were 2 patients in each gender who had been allocated to group 1 before surgery but ended up belonging to group 2 because the PPCR turned out to not be 0%, owing to correction of patellar maltracking during surgery. Furthermore, the discrepancy eventually increased after the inclusion and exclusion criteria of this study were defined. The Kujala scores, IKDC scores, Tegner activity scores, congruence angles, and PPCRs before and after surgery and the time needed to sit squarely, rates of tightness and crepitus in the knee joint after surgery, and rate of osteochondral fracture of the patella in each group are presented in Table 2.
6
J. SUGANUMA ET AL.
Table 2. Results of Assessments of MPFL Reconstruction Group 1 (n ¼ 18) Clinical findings Kujala score Before surgery 2 yr after surgery IKDC score Before surgery 2 yr after surgery Tegner activity score Before surgery 2 yr after surgery Time needed to sit squarely, mo Tightness in knee joint Crepitus in PFJ Radiographic findings Congruence angle, Before surgery 3 mo after surgery 2 yr after surgery Arthroscopic findings Osteochondral fracture of patella PPCR with ER and ESQ, % BR AR
Group 2 (n ¼ 28)
P Value
1 b (a ¼ .05)
75.2 13.4* 96.4 2.3*
73.1 10.9* 97.7 2.3*
.57 .075
.086 .43
69.8 12.4* 82.8 13.1*
69.3 10.7* 91.3 9.1*
.87 .012
.053 .73
4.6 (3 to 7) 4.6 (3 to 7) 4.6 2.7 4 (22%) 5 (28%)
4.6 (3 to 7) 4.6 (3 to 7) 1.8 1.4 0 (0%) 1 (4%)
.96 .78 .001 .019 .028
.050 .057 .99 .77 .68
22.8 20.6* 14.4 5.6 14.0 8.9*
26.8 22.6* 4.1 15.2 3.4 16.2*
.55
.091
.007
.70
6 (33)
10 (36)
>.99
102.1 10.9* 0.0 0.0*
103.4 10.9* 5.3 2.2*
.69 <.001
.053 .067 >.99
NOTE. Data are presented as mean standard deviation, mean (range) or number (percent). The P value represents the comparison between the groups. AR, after reconstruction of MPFL; BR, before reconstruction of MPFL; ER, external rotation of knee joint; ESQ, electrical stimulation of quadriceps muscle; IKDC, International Knee Documentation Committee subjective evaluation; MPFL, medial patellofemoral ligament; PFJ, patellofemoral joint; PPCR, position of patellar central ridge.17 *Significant difference (P < .001) between before surgery and 2 years or more after surgery or between BR and AR.
In group 1, 6 patients had an osteochondral fracture on the medial facet of the patella, 9 had grade I or II degenerative change of the patellar cartilage, and 9 had grade III or IV degenerative change. The apprehension sign was seen in all patients, but it disappeared after surgery (P < .001). The J-sign was apparent in 6 patients, but it disappeared after surgery (P ¼ .031, b ¼ .21). There were significant improvements in the Kujala score, IKDC score, congruence angle, and PPCR after surgery (all P < .001). However, there was no significant difference in the Tegner activity score between before and after surgery (P ¼ .74, b ¼ .94) or the congruence angle between 3 months and 2 years or more after surgery (P > .99). Complications included a case of paresthesia of the infrapatellar region, but this resolved completely in 2 months. In group 2, 10 patients had an osteochondral fracture on the medial facet of the patella, 13 had grade I or II degenerative change of the patellar cartilage, and 15 had grade III or IV degenerative change. All patients had the apprehension sign before surgery, and it disappeared after surgery (P < .001). The J-sign was apparent in 8 patients, and only a trace of the J-sign remained in 1 patient after surgery (P ¼ .016, b ¼ .18). There were significant improvements in the Kujala score, IKDC score, congruence angle, and PPCR after
surgery (all P < .001). However, there was no significant difference in the Tegner activity score between before and after surgery (P ¼ .71, b ¼ .94) or the congruence angle between 3 months and 2 years or more after surgery (P > .99). Complications included 2 cases of paresthesia of the infrapatellar region, but each remitted completely within 2 months. There were no significant differences before surgery in the Kujala score, IKDC score, Tegner activity score, congruence angle, and PPCR between the 2 groups. Moreover, there were no significant differences in the rates of the apprehension sign, J-sign (P ¼ .75, b ¼ .94), degenerative change of the patellar cartilage (P ¼ .82, b ¼ .94), or osteochondral fracture of the patella before surgery. However, the IKDC score and the congruence angle became significantly larger in group 2 than in group 1 (P ¼ .012 and P ¼ .007, respectively) after surgery. The time needed to sit squarely was significantly shorter in group 2 than in group 1 (P ¼ .001), and the rates of tightness and crepitus in the knee joint were significantly higher in group 1 than in group 2 (P ¼ .019 and P ¼ .028, respectively). No significant differences between the 2 groups remained in the Kujala score, Tegner activity score, and J-sign (P > .99, b ¼ .88) after surgery. There were no patients with decreased ROM of the knee joint, progression of osteoarthritis on radiographs, an episode of lateral or medial
RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
instability of the patella, wound infection, fracture of the patella, or graft tear in either group.
Discussion The most important finding of this study is that subjective evaluations are better in knee joints in which the patella is repositioned slightly lateral to the center of the trochlea than in those in which the patella is reduced to the strict center when recurrent dislocation of the patella is treated with MPFL reconstruction using a synthetic graft. In both groups, there was no recurrence of patellar instability, restriction of ROM of the knee joint, or the apprehension sign in all patients after surgery, and the mean values of the congruence angle, Kujala score, and IKDC score improved significantly (all P < .001). There were no significant differences between the 2 groups in the mean values of the Kujala score and IKDC score before surgery and in the Kujala score after surgery. However, the mean value of the IKDC score was significantly higher in group 2 than in group 1 after surgery (P ¼ .012). The reason for the discrepancy between the Kujala score and the IKDC score in the postoperative results seems to be that the latter provided a more precise subjective evaluation, especially among patients who recovered fairly well, whereas the former provided a more objective evaluation, although both scales have sufficient responsiveness.21,27 When the patella was reduced slightly lateral to the center of the trochlea, leaving room for the patella to translate medially, tension of the reconstructed MPFL was not high and medial patellofemoral contact pressure was 0 MPa or close to 0 MPa at 30 of flexion of the knee joint. However, when the patella was reduced to the strict center of the trochlea, patients took a longer time to be able to sit squarely (P < .001); the rate of patients who felt tightness in the knee joint increased (P ¼ .019) during activity related to deep flexion of the knee joint, especially extending the knee joint from deep flexion; and the rate of knee joints that had crepitus in the patellofemoral joint increased (P ¼ .028). Because we used a synthetic graft, which is much stiffer than the native MPFL, there is potential for high tension of the reconstructed MPFL and high contact pressure of the medial patellofemoral joint in this group,14,28 even though medial patellar subluxation or patellar maltracking was not recognized during surgery. Moreover, all of the patients who had crepitus in the patellofemoral joint had an osteochondral fracture on the medial facet of the patella. They recognized the crepitus after surgery, the extent of which showed a slight tendency to decrease with time. Although there was no significant difference in the rate of osteochondral fracture of the patella between the groups (P > .99), the rate of crepitus was significantly higher in group 1 than in group 2 (P ¼ .028), which might show that this discrepancy was attributed to the difference in
7
contact pressure on the fracture site between the groups. Therefore, the strict center of the trochlea, which seemed to be the ideal reduced position for the recurrently dislocated patella at first sight, overlapped the critical zone where the morbidity rate of tightness in the knee joint and crepitus in the patellofemoral joint increased, and to obtain a better subjective evaluation, it would be better not to reduce the patella to the strict center of the trochlea using the synthetic graft. Graft tensioning is reported to be an important factor in keeping the patella centered in the trochlear groove without overloading the patellofemoral joint.6 In clinical studies, the risk of undertensioning the graft has been balanced with the risk of overconstraining the patellofemoral joint. Because the patella engages into the trochlea at 30 of flexion, fixation of the graft at 45 of flexion with no tension would avoid overconstraining the graft.5 The reconstructed MPFL should allow 2 to 3 patellar quadrants of translation, without overly constraining the patella.6 In cadaveric studies using semitendinosus tendon grafts, Beck et al.15 reported that tension of 2 N applied to an MPFL reconstruction stabilized the patella without increasing medial patellofemoral contact pressure and that 10 N restricted lateral patellar translation and inappropriately redistributed patellofemoral contact pressures. Stephen et al.,18 using a double-strand gracilis tendon graft, reported that graft tension of 2 N restored intact medial and lateral joint contact pressure and patellar tracking but graft tension of 10 or 30 N caused significant increases in medial pressure and tilt. In contrast, Philippot et al.16 reported that 10 N of graft tension was acceptable to restore normal patellar tilt, lateral shift, and rotation and that more than 20 N induced a major overcorrection. However, because the tightness of the lateral retinaculum and the surface geometry of the trochlea and patella vary among patients, it seems difficult to achieve expected patellofemoral congruence by applying a determined tension to the graft. Therefore, we have tried to obtain the expected patellofemoral congruence under arthroscopic control by changing the length of the reconstructed MPFL at 30 of flexion, where the patella is most vulnerable.29,30 However, if the reconstructed MPFL is slack at the time of acquisition of the expected congruence, recurrence of patellar instability would occur. It has been reported that a recurrently dislocated patella was shifted laterally at the maximum with ER and ESQ performed with the knee joint flexed 30 .24 Under such conditions, therefore, we verified the patellofemoral congruence on arthroscopy before the graft was fixed to the femur. The reason we used ER and ESQ instead of applying a laterally directed external force on the patella to prevent slackness of the graft was that we wanted not only to keep the patella shifted as laterally and physiologically as possible but also to confirm how
8
J. SUGANUMA ET AL.
the patella changed its dynamic behavior after MPFL reconstruction. In knee joints without patellar disorders, ESQ developed a tendency to shift the patella to the center of the trochlea with ER.24 In knee joints with patellar instability, on the other hand, ESQ shifted the patella far lateral on the trochlea with ER before MPFL reconstruction and still exerted a laterally directed force on the patella in all patients immediately after the reconstruction, which might suggest that a stiffer or stronger graft may be warranted for the reconstruction.28 There was no significant difference in the congruence angle between 3 months and about 4 years after surgery in this study. However, Kita et al.31 reported that the congruence angle improved immediately after MPFL reconstruction using a double-looped semitendinosus tendon but it returned toward the preoperative value to some extent about 1 year after surgery. Zhao et al.,32 reconstructing the MPFL using a looped semitendinosus tendon, also observed that correction of the congruence angle deteriorated over time, especially in the first 12 postoperative months. Furthermore, Xie et al.,33 comparing the results of MPFL reconstruction using a looped semitendinosus tendon with polyester suture augmentation and that without augmentation, reported that deterioration of the corrected position of the patella was not observed in the former whereas slackness of the reconstructed MPFL increased gradually, especially in the first 24 months, in the latter. They speculated that the reconstructed graft might undergo graft remodeling or stress relaxation.31-33 If a reconstructed MPFL becomes slack, and if the extent of the slackness varies unpredictably by case, it would be difficult to attain consistent results with MPFL reconstruction. One of the reasons the reconstructed MPFL did not become slack in this study was the use of a synthetic graft for the operation. The theoretical strength of the reconstructed MPFL in the present method is as much as 2,100 N, which is about 10 times higher than that of the native MPFL29,34 and about 5 times higher than that of a tendon graft.35 Limitations There are several limitations to this study. First, the number of patients was not sufficient to verify a significant difference in the Kujala score between the 2 groups. When the required sample size was computed under the conditions of a ¼ .05, power of 0.80, d (difference in population means) of 5, s (within-group SD) of 6.4,32,33 and allocation ratio of 1, the required sample size was 27 (total of 54), showing that this study did not have enough patients. However, the mean Kujala scores of both groups were high, and the difference in mean values between the groups was very small. Therefore, even if a significant difference in the Kujala
score had been obtained by increasing the number of patients, the difference would have resulted in less than 2 points. Second, the intraclass correlation coefficients of group 2 for intraobserver and interobserver reliability analyses of the PPCR were 0.77 and 0.64, respectively, which were lower than those previously reported (0.93 and 0.87, respectively).24 The reason for the low reliability of PPCR in this study seems to be that the shape of the patellar central ridge was deformed in some patients and it was difficult in such cases to identify the central ridge accurately using only still video frames. During arthroscopic examination, however, it was possible to identify the central ridge (C in Fig 1) in all cases by pushing the patella from the anterior direction to reduce the patella to the strict center of the trochlea temporarily. Because the deepest point of the trochlea (D in Fig 1) is easy to identify on arthroscopy, the point on the patellar articular surface touching point D is determined to be the patellar central ridge. Third, all operations and measurements of PPCR were performed by the same surgeon, and evaluation of patients was performed at our outpatient clinics mostly by the same surgeon. Therefore, this study was not blinded. Fourth, the femoral anchoring site of the reconstructed MPFL in this study was deviated slightly posteriorly. Because the patella with recurrent dislocation is most vulnerable in the first 30 of flexion,29,30 we thought that, if the reconstructed MPFL became tight around 30 of flexion, it was allowed to be loose at more than 60 of flexion.36,37 With this idea, in our study the femoral anchoring site was deviated posteriorly, which is, however, within the range of the reported area for the anchoring site,10,38-42 to meet the demand of all our patients to sit squarely after surgery. Fifth, whether the J-sign reproduced by ESQ can substitute for the natural J-sign is unknown, because ESQ differs from voluntary contraction of the quadriceps. However, the only knee joints in which the J-sign was detected on clinical examination so far were able to reproduce the J-sign with ESQ during surgery. Sixth, the operative devices were changed for small-sized ones in 2007, which eventually made skin incisions slightly smaller. A small, flat metal electrode to stimulate the quadriceps was also changed to a 23-gauge Cattelan needle (TOP, Tokyo, Japan) in 2007. Because the changed devices and electrode were functionally identical to the previous ones, these changes during this study did not appear to have compromised its results. Seventh, a PT20 was used for MPFL reconstruction, and the theoretical linear stiffness of the graft is 220 N/mm, which is about 7 times stiffer than the native MPFL34 and about 2.5 times stiffer than a tendon graft.35 Therefore, if a tendon graft is used in a study of the same design, different results might be obtained, because a less stiff graft may be more forgiving of over-tensioning that would occur in reduction to the strict center of the trochlea.
RECONSTRUCTION OF MPFL WITH ARTHROSCOPY
Conclusions When recurrent dislocation of the patella was treated with MPFL reconstruction using a synthetic graft, subjective evaluations were better in knee joints in which the patella was repositioned slightly lateral to the center of the trochlea than in those in which the patella was reduced to the strict center, although there was no significant difference in knee function between them.
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