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Medialization of the patella in total knee arthroplasty
The Journal of Arthroplasty Vol. 12 No. 2 1997
M e d i a l i z a t i o n of t h e Patella in Total K n e e A r t h r o p l a s t y Kris Lewonowski, MD,* Lawrence D. Dorr, MD,J- Edward J. McPherson, MD,JGlen Huber, J- and Zhinian Wan, MD]-
Abstract: Patellar complications of total knee arthroplasty remain the most common
cause of pain and reoperation. Laboratory studies have suggested that medialization of the patella will improve tracking of the patella on the trochlea of the femoral component. The purpose of this study was to determine if clinical medialization of the patellar component on the patellar bone would improve tracking of the patella as demonstrated radiographically. Sixty-two knees were randomized so that 31 knees had a centrally placed patellar component and 31 had the patellar component placed on the medial two thirds of the patellar bone. There was no difference between the two groups with respect to either clinical or radiographic results in the first year after surgery. There was no improvement compared with previous reports in the incidence of tilt and displacement. The one improvement was a reduction in the incidence of lateral release. Thus, consequences of lateral release such as postoperative morbidity, avascular necrosis of the patella, and stress fracture of the patella can be avoided. It is recommended that the patellar component be placed on the medial two thirds of the patella to reduce the occurrence of lateral release. Tracking of the patella during surgery can be assessed using a single suture placed at the superior pole of the patella, and this technique in combination with the no-thumbs test provides an additional means of evaluation for patellar tracking. Key words: total knee arthroplasty, medialization, patella, lateral release.
The most c o m m o n cause of pain following total knee arthroplasty (TKA) is extensor m e c h a n i s m imbalance and poor tracking of the patella [1-4]. M a n y factors influence patellar tracking, including prosthetic design and surgical technique [5-10]. Resurfacing of the patella has b e e n s h o w n to significantly i m p r o v e pain relief obtained with TKA [2,3,11]; however, resurfacing of the patella increases the risk for complications, with subluxation of the patella being the most c o m m o n [3,4, 11,12]. In an effort to i m p r o v e patellar tracking m a n y authors have a d v o c a t e d the use of lateral
retinacular release [4,8,12-15]. Unfortunately, lateral retinacular release c a n n o t be p e r f o r m e d w i t h o u t complications, w h i c h include avascular necrosis, stress fracture, w o u n d slough, prol o n g e d w o u n d healing, a n d an increased incidence of infection [2,4,16,17]. Medialization of the patellar c o m p o n e n t on the patellar b o n e has also b e e n suggested as a m e a n s of i m p r o v i n g patellar tracking [ 5 , I 0 , 1 3 , 1 4 , 1 8 ] ; h o w e v e r , the only studies of the direct effect of a medial position of the patellar c o m p o n e n t o n patellar tracking a n d lateral retinacular t e n s i o n were perf o r m e d on cadaveric specimens [ 1 0 , I 9 ] . The h y p o t h e s i s of this study is that in TKA, medialization of the patellar c o m p o n e n t as c o m p a r e d with a central position i m p r o v e s postoperative patellar tilt, displacement, a n d the n e e d for lateral release.
From the *Kansas Orthopedic Center, Wichita, Kansas, and tUSC Center for Arthritis and Joint Implant Surgery, Los Angeles, California. Reprint requests: Lawrence D. Doff, MD; USC Center for Arthritis and Joint Implant Surgery, 1500 San Pablo Street, 7th Floor, Los Angeles, CA 90033.
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Materials and Methods B e t w e e n October I993 and July 1994, at the University of S o u t h e r n California Center for Arthritis and Joint Implant Surgery, primary TKA was performed in 74 knees in 74 patients. Seven patients were not included in this study because of the use of a prosthetic design different from that used for this study. Sixty-seven knees were assigned to have either a central or medial position of the patellar c o m p o n e n t d e p e n d e n t on w h e t h e r their hospital record n u m b e r was odd or even. Five knees that were assigned a medial position of the c o m p o n e n t had patellas that during surgery were f o u n d to be too small for this technique and were therefore excluded from the statistical comparison w h i c h was accomplished b e t w e e n the remaining 62 knees. Thirty-one knees!lhad a central patella and 31 a medial position o~ the patellar component. Demographics are listed in Table 1. The average age of the entire group was 67 years. All knees were operated with the Apollo cruciateretaining TKA system (Intermedics Orthopedics, Austin, TX). The Apollo TKA is a symmetric condylar design (Fig. 1). The femoral c o m p o n e n t with a symmetric trochlea is manufactured from cobalt-chrome metal. The tibia has a congruent dished articulation surface and was implanted either as an all-polyethylene tibia or as a modular insert in a stemmed cobalt-chrome component. The prosthetic patella is manufactured from ultra highmolecular-weight polyethylene. The shape is an
Fig. 1. (A) The Apollo cruciate-retaining knee is pictured here with the all-polyethylene tibial component. In the knees in this study, both the all-polyethylene tibial component and the metal-backed tibial component were used. (B) The trochlea is a symmetric design with a deepened patellar groove. The patella is an elliptical dome. There is a congruent fit of patella and trochlea.
Table 1. Demographics Categories
Central
Iganees 31 Women 19 Age (y} 66(39-80) Weight (lb) 186 Osteoarthritis 29 Rheumatoid arthritis 2 Follow-up period (too) 12
Medial 31 19 67(34-82) 173 28 3 12.2
P Value 1.00" 1.00" 1.00~ 0.73t 0.64" 0.75t
*Chi-square test. ~-t-test.
elliptical dome for all sizes but the smallest, which is a r o u n d dome shape. Four sizes of patellar component were available: 29 x 29 mm, 29 x 32 mm, 32 x 35 mm, and 35 x 38 m m w h i c h were oval dome shaped. Four sizes of the patellar c o m p o n e n t were used in this study: one 29 x 29 mm, thirty 29 x 32 mm, twenty-four 32 x 35 mm, and seven 35 × 38 mm. The three smallest components are 7 m m thick at the dome and 3.5 m m thick at the edge. The 35 x 3 8 - m m size is 8.5 m m thick at the dome. All operations but one were performed by one surgeon (L.D.D.); roentgenographic m e a s u r e m e n t s were performed by two i n d e p e n d e n t observers. Data were collected by two observers i n d e p e n d e n t of the surgeons, one of w h o m had participated in the radiographic measurements. The technique at surgery included the use of a t o u r n i q u e t unless the patient had diabetes or there were absent pulses in the popliteal fossa or distally. A medial parapatellar a r t h r o t o m y was used in all
Medialization of Patella inTKA
knees which e x t e n d e d from the top of the quadriceps t e n d o n to the medial aspect of the tibial tubercle. The quadriceps t e n d o n was incised so that it remained almost completely intact, with a 5 - m m strip of medial t e n d o n left attached to the vastus medialis muscle for t e n d o n repair at closure. The infrapatellar fat pad was partially excised to allow adequate exposure of the lateral tibial plateau. The patella was everted laterally and held by the use of a Kocher clamp placed on the quadriceps t e n d o n superiorly and the patellar ligament inferiorly. The patella was cut w i t h o u t the use of a jig from the medial to the lateral subchondral bone. The thickness of the patella was judged by palpation to be symmetric with equal bone medial, lateral, superior, and inferior. The level of the cut was n e v e r below the insertion of the quadriceps t e n d o n or patellar ligament. In the central patella three holes were drilled t h r o u g h a template placed centrally and a patellar c o m p o n e n t was selected that w o u l d resurface most of the patellar bone and result in a patellar height of 20 to 25 ram. In the medial components the drill holes were placed medially on the cut surface of the patella and a patellar c o m p o n e n t was selected that w o u l d leave one third of the lateral patellar bone uncovered. If the patient bone was too small to permit only two-thirds coverage with the smallest c o m p o n e n t , the patella was excluded from the study group. Intramedullary instrumentation was used for the f e m u r and tibia. The desired valgus position of the femoral compon e n t was 4 ° to 6 ° . The femoral c o m p o n e n t was rotationally aligned with the apex of the lateral epicondyle which routinely required external rotation of 3 °. The sizing of the femoral c o m p o n e n t did not usually permit it to be placed laterally. Tibial rotation was routinely internal, with the plastic eminence aligned with the medial third of the tibial tubercle. Fixation in each knee was accomplished by the use of b o n e - c e m e n t for the femoral, tibial, and patellar components. Patellar tracking was evaluated with the tourniquet inflated, and lateral release, w h e n necessary, was p e r f o r m e d with the t o u r n i q u e t inflated. Patellar tracking was confirmed with the t o u r n i q u e t deflated, immediately after c e m e n t fixation was completed. The decision to perform a lateral retinacular release was based on the tracking of the patellar c o m p o n e n t using a single-suture technique. This technique is a modification of the not h u m b s test described by Scott [I8,20]. With the n o - t h u m b s test, the patefla should track centrally in the trochlear groove w i t h o u t any m a n u a l support. With the single-suture technique, tilt or subluxation that occurs with the n o - t h u m b s test is
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eliminated w h e n a single suture of zero strength is placed to reattach the quadriceps t e n d o n at the superior pole of the patella. Contact of the patella had to be complete t h r o u g h o u t flexion. If this single suture breaks during flexion of the knee to 130 °, t h e n the extensor m e c h a n i s m is too tight and lateral release is necessary. If the suture does not break t h r o u g h full flexion of the knee, t h e n a lateral release is not necessary. If a lateral retinacular release was performed, no attempt was made to isolate and protect the superior-lateral geniculate artery. The lateral release was p e r f o r m e d by an incision from the tibial bone obliquely to the superior pole of the patella. This means that the entire lateral retinacular incision is distal to the lateral geniculate artery and does not require exposure of the lateral geniculate artery. No attempt was made to isolate and protect the superior-lateral geniculate artery during this release. The capsule was closed with interrupted sutures with the knee in extension. No intra-articular drains were placed in any of the 62 TKAs included in this study. All patients had epidural anesthesia and some also had general anesthesia. An indwelling epidural catheter was used for m o r p h i n e administration for 48 hours after surgery. In the recovery room, all patients had their operated extremity placed in a continuous passive m o t i o n machine with a range from full extension to 90 ° of flexion. The continuous passive m o t i o n machine was used only during a I 2 - h o u r period from 7:00 a.m. to 7:00 p.m. w h e n the patient was in bed. Several hours each day, the patient was out of bed participating in physical t h e r a p y or undergoing i n d e p e n d e n t activity. Some patients could not tolerate flexion to 90 ° and the range of passive m o t i o n was decreased to a comfortable level and gradually increased to 90 ° degrees over the succeeding 2 to 3 days. The continuous passive m o t i o n machine was continued until the patient had obtained active flexion of 80 ° for 24 hours or for a m a x i m u m of 5 days. Patients were discharged from the acute ward w h e n they had achieved i n d e p e n d e n c e in transfer and ambulation and achieved active range of m o t i o n of at least 15 ° to 80 °. Patients w h o had not achieved this level of function or w h o were not confident in their ability to live i n d e p e n d e n t l y at h o m e by the fifth postoperative day were transferred to the rehabilitation unit. After surgery, patients were evaluated at 6 weeks, 3 months, 6 months, and 1 year. At each visit, the range of m o t i o n of the knee was determined; this included the presence of extensor lag or flexion contracture and active extension and flexion. Before surgery and 6 m o n t h s afterward,
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patients w e r e evaluated by The Knee Society clinical rating system [21]. Before surgery and at each postoperative visit, roentgenographic examination of the knee included weight-bearing anteroposterior a n d lateral roentgenographs, as well as a lateral radiograph at 30 ° to 45 ° of flexion. The patella was further evaluated with a 45 ° M e r c h a n t ' s or sunrise view of the k n e e [22] (Fig. 2). The patellar view was p e r f o r m e d with a standard f r a m e to hold the k n e e flexed 45 ° and to support the cassette at the patient's ankle. The radiographic b e a m is directed at 60 ° to be tangential to the patella. Patellar tilt and displacement w e r e m e a s u r e d by the m e t h o d s described by Bindelglass et al. [23]. Preoperative patellar tilt was defined as the angle b e t w e e n a line f r o m the anterior limits of the femoral condyles and the equatorial line of the patella. After surgery, a line d r a w n t h r o u g h the patellar b o n e - p r o s t h e s i s interface was used to replace the equatorial patellar line. Patellar disp l a c e m e n t was m e a s u r e d as the distance b e t w e e n the center of the patella a n d a line d r a w n t h r o u g h the center of the femoral condyles. Final tilt and displacement w e r e m e a s u r e d f r o m a tangential pateflar radiograph obtained at the 6 - m o n t h postoperative visit (Fig. 3).
Fig. 3. The patella shows displacement of 8 ram. The angle is 5° so that it is not an abnormal tilt. The line drawing shows the technique used to measure the tilt and displacement of the patella. This patella has a medialized patellar component.
All data w e r e analyzed statistically using SYSTAT software (SPSS, Chicago, IL). The chi-square test was used for c o m p a r i s o n of sex, patellar tilt, a n d lateral release. Student's t-test was used for comparison of Knee Society scores, range of motion, and a n a t o m i c axis.
Results Sixty-two knees were included in this study; there were 19 w o m e n and 12 m e n in each group. Thirtyone knees had a central patella and 31 a medialized patella. The average age of the 62 patients was 67 years (range, 34--82 years). The duration of postoperative evaluation averaged 12 m o n t h s (range 6-15 months). Demographics for the two groups are listed in Table 1. B e t w e e n the central and medial groups there was not a n y statistical difference for sex, age, weight, diagnosis, side of the operated extremity, or length of follow-up period. The average p r e o p e r a t i v e Knee Society k n e e score for the central group was 46 and at last foll o w - u p visit was 87. For the medial group the average p r e o p e r a t i v e Knee Society k n e e score was 48 and at last follow-up visit was 90. Range of m o t i o n data are listed in Table 2. The distribution of the p r e o p e r a t i v e angulation of the knees is given in Table 3. After surgery, the
Table 2. Range of Motion
Preoperative 3 Months Fig. 2. The Merchant's view is shown with a patella that is centralized after surgery. The radiographs have been computer-enhanced to facilitate the bony outline.
Central Medial
110+15 110-+16
102_+14 101-+12
6 Months
Final
110_+14 115_+14 111-+ 11 117_+11
Medialization of Patella in TKA Table
3. Preoperative Angulation CO”
Central Medial
13 13
o”-10”
>loO
14 15
4 3
average anatomic axis for the central group was 4.6” (range, -2” to ll’), and that for the medial group was 4.1” (range, -3” to 13”). There was no statistical difference between the anatomic are of the central and medial groups before surgery, after surgery, or at the last follow-up examination. The average thickness of the patellar cut was 7.9 mm in the central group and 8.1 mm in the medial group (P = .61). There was no difference in patellar thickness because the patellar component thickness was constant at 7 mm in all but the 7 knees that had the 35 x 38-mm size. The patellar cut was symmetric in 22 of 31 in the central group and 23 of 31 in the medialized group (P = .78). Preoperative patellar tilt was observed in 10 knees of the central group and 12 knees of the medial group. After surgery, 4 of the 10 in the central group and 5 of the 12 in the medial group still demonstrated patellar tilt (Fig. 4). The average tilt angle for the central group decreased from 10.6” before to 8.3” after surgery, and the average tilt angle for the medial group decreased from 12” to 1 lo. Overall, of the 62 knees, 22 had postoperative tilt. Twelve knees that did not have preoperative tilt had postoperative tilt. Fifteen of the 22 knees had lateral tilt and 7 had medial tilt. Preoperative
Fig. 4. The top radiograph shows a knee with preoperative tilt. The bottom view shows the postoperative radiograph with the same tilt. There is statistical correlation to tilt after surgery when it is present before surgery.
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patellar displacement was observed in 5 knees in the central group and 10 knees in the medial group. After surgery, all 5 of the central group and 6 of the 10 in the medial group still demonstrated displacement (P = .74). In the central group the average displacement increased from 6.7 to 9.3 mm and, in the medial group, from 8.9 to 9.8 mm. Overall, 37 of 62 knees had postoperative displacement of at least 5 mm. Twenty-five knees that did not have preoperative displacement had postoperative displacement. Preoperative patellar tilt and displacement were observed in 4 knees in the central group and 6 knees in the medial group. After surgery, this increased to 7 knees in the central group and 9 knees in the medial group. Lateral release was required for satisfactory intraoperative tracking of the patella in 15 of 3 1 knees in the central group and in 4 of 31 knees in the medial group (P < .002). Of the 5 knees with the patella too small for medialization, 3 had a lateral release. In one patient in the medial group the lateral uncovered portion of the patella fractured while the patient was playing golf 4 months after surgery. A nondisplaced vertical fracture subsequently healed and the patient became clinically asymptomalic with a 12-month Knee Society score of 90. This patient did not have a lateral release at the time of surgery. Discussion The hypothesis in this study was that medial placement of the patellar component covering only the medial two thirds of the patellar bone would improve postoperative patellar tilt, displacement, and the need for lateral release. Medial placement of the patellar component in the laboratory resulted in statistically reduced lateral tracking and subluxation compared with a central position [lo]. Yoshii et al, also suggested that medial position of the patellar component decreased the Q-angle and decreased the lateral shear force on the patella [lo]. Patellar tilt and displacement from the patellofemoral groove occur commonly on radiographs of clinically satisfactory TICAs [9,23]. Bindelglass et al. reported incidences of 31.2% patellar tilt and 14.5% patellar displacement in clinically successful TICAs [23]. In that study, there was no change in patellar position 3 months after surgery. Therefore, the results measured in this study would not change with longer follow-up evaluation. In this study, we did not prove the hypothesis that medial displacement of the patellar component to cover only the medial two thirds of the cut patellar bone would decrease the incidence of patellar tilt
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and displacement. Forty-three percent of patients with preoperative tilt and 73% of patients with preoperative displacement continued to have the same tilt and displacement after surgery. This finding is in agreement with that of BindeIglass et al., w h o found a statistical correlation to the incidence of preoperative and postoperative tilt [23]. As was reported by Bindelglass et al., there is no correlation to the radiographic findings of tilt and displacement and clinical symptoms reported by the patients [23]. Although satisfactory patellar tracking was achieved in each of these patients during surgery, this intraoperative balance is only static balance. After surgery, the forces on the patella are both dynamic and static, which explains the inability during surgery to predict postoperative tilt and displacem e n t except in patients w h o exhibit this type of patellar tracking before surgery. The trochlea of the Apollo knee design used in these patients is symmetric. Both laboratory and clinical results have s h o w n n o difference with a symmetric or asymmetric trochlea in the tracking of the patella. In the study by Bindelglass et al., there was n o difference b e t w e e n the symmetric and asymmetric trochleas [23]. In a laboratory study by Petersilge et al., there was n o significant difference b e t w e e n the two shapes [24]. The patellar design used was an oval d o m e (Fig. 1). Perhaps a patella that was eccentric in shape so that it was thicker medially and t h i n n e r laterally w o u l d be as effective as medial displacement of this oval dome. Briard and H u n g e r f o r d reported that this eccentric-shaped patella p e r f o r m e d well in a clinical series [18]. C o n g r u e n c y of the patella to the trochlear groove w o u l d seem to be important. Creep of the p o l y e t h y l e n e of the patella does occur to p r o m o t e better tracking [9] so that the absolute importance of c o n g r u e n c y has not b e e n demonstrated. The hypothesis that the incidence of lateral release would be reduced with medial displacem e n t of the patellar c o m p o n e n t was proved. Lateral release was required in 48% of the central group and 13% of the medial group (P = .002). Lateral retinacular release is a c o m m o n technique employed to improve patellar tracking at the time of surgery. The question still u n a n s w e r e d is w h a t would be the incidence of patellar tilt and subluxation if no lateral releases were performed. In this study, we used the single-suture technique to determine satisfactory intraoperative patellar tracking. The single-suture technique is less severe t h a n the n o - t h u m b s test, which has b e e n c o m m o n l y used. With the single-suture technique, we still p e r f o r m e d a lateral retinacular release in 19 (31%)
knees. The incidence of tilt and displacement is not i m p r o v e d in knees that had lateral release as c o m p a r e d with those that did not [23]; however, Ritter and Campbell reported that lateral release p r e v e n t e d patellar tilt recognizable at surgery and c o n t i n u e d to provide postoperative patellar stability superior to that of those knees that had no release [t6]. Lateral retinacular release is assodated with complications which include avascular necrosis and stress fractures [3,I7,25-27]. Wetzner et al. found that lateral retinacular release resulted in decreased radionuclide uptake after surgery and stated the fat pad was important in preserving the blood supply to the patella {27]. M c M a h o n et al. used a scintigraphic technique to measure a higher incidence of vascular compromise of the patella after a lateral retinacular release and reported that excision of the infrapatellar fat pad did not compromise patellar vascularity [28]. In this study, the single advantage of medial displacement of the patellar c o m p o n e n t on the cut patellar bone is the reduction and r e q u i r e m e n t of lateral retinacular release for satisfactory intraoperative patellar tracking. Avoidance of lateral retinacular release does reduce the incidence of avascular necrosis and stress fractures of the patella and does reduce the postoperative morbidity of lateral knee ecchymosis and swelling and pain. Medial displacem e n t of the patellar c o m p o n e n t is not always possible because of the size of the patellar bone. In this study, in five knees the patellar c o m p o n e n t could not be displaced medially because the patellar bone was too small to permit this technique. The results in this study would suggest that satisfactory clinical tracking of the patella can be obtained with external rotation of the femoral c o m p o n e n t , alignment of the eminence of the tibia with the medial third of the tibial tubercle, and medial displacement of the patellar c o m p o n e n t to cover the medial two thirds of the cut patellar bone. Radiographic measurements of patellar position were not affected by the medial displacement of the patellar component. The single-suture technique for assessment of intraoperative patellar tracking was successful and allows an additional criterion to the not h u m b s test for evaluation of patellar stability.
References 1. Dorr LD, Boidaro RA: Technical considerations in total knee arthroplasty. Clin Orthop 205:5, 1986 2. Boyd AD, Ewald FC, Thomas WH et al: Long-term complications after total knee arthroplasty with or without resurfacing of the patella. J Bone Joint Surg 75A:674, I993
Medialization of Patella inTKA
3. Clayton ML, Thirupathi R: Patellar complications after total condylar arthroplasty. Clin Orthop 170:152, 1982 4. Rand JA: Current concepts review: the patellofemoral joint in total knee arthroplasty. J Bone Joint Surg 76A:612, 1994 5. Figgie I-IE, Goldberg YM, Heiple KH et al. The influence of tibial-patellofemoral location on function of the knee in patients with the posterior stabilized condylar knee prosthesis. J Bone Joint Surg 68A:1035, 1986 6. Grace JN, Rand JA: Patellar instability after total knee arthroplasty. Clin Orthop 237:184, 1988 7. Lotke PA, Echer ML: Influence of positioning of prosthesis in total knee replacement. J Bone Joint Surg 59A:77, 1977 8. Merkow RL, Soudry M, Insall JN: Patellar dislocation following total knee replacement. J Bone Joint Surg 67A:1321, 1985 9. Ranawat CS: The patello-femoral joint in total condylar knee arthroplasty: pros and cons based on five to ten year follow up observations. Clin Orthop 205:93, 1986 10. Yoshii I, Whiteside LA, Anouchi YS: The effect of patellar button placement and femoral component design on patellar tracking in total knee arthroplasty. Clin Orthop 275:211, 1992 11. InsalI J, Scott WN, Ranawat CS: The total condylar knee prosthesis: a report of two h u n d r e d and twenty cases. J Bone Joint Surg 61A:173, 1979 12. Johnson DE Eastwood DM: Lateral patellar release in knee arthroplasty: effect on w o u n d healing. J Arthroplasty 7 (suppl):427, 1992 13. Brick GW, Scott RD: The patellofemoral component of total knee arthroplasty. Clin Orthop 231:163, 1988 14. Insall JN, Haas SB: Complications of total knee arthroplasty, p. 905. In Insall JN (ed): Surgery of the knee. 2nd ed. Churchill Livingstone, New York, 1993 15. Ritter MA, Keating ME, Faris PM: Clinical, roentgenographic, and scintigraphic results after interruption of the superior lateral genicular artery during total knee arthroplasty. Clin Orthop 248:145, 1989
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16. Ritter MA, Campbell ED: Postoperative patellar complications with or without lateral release during total knee arthroplasty. Clin Orthop 219:163, 1987 17. Scuderi G, Scharf SC, Meltzer LP, Scott WN: The relationship of lateral releases to patella viability in total knee arthroplasty. J Arthroplasty 2:209, 1987 18. Briard JL, Hungerford DS: Patellofemoral instability in total knee arthroplasty. J Arthroplasty 4(suppl): 87, 1989 19. Bayley JC, Scott RD: Further observations on metalbacked patellar component failure. Clin Orthop 236:82, 1988 20. Scott RD: Prosthetic replacement of the patellofemoral joint. Orthop Clin North Am 10:129, 1979 21. Insall JN, Dorr LD, Scott RD, Scott WN: Rationale of The Knee Society clinical rating system. Clin Orthop 248:13, 1989 22. Merchant AC, Mercer RL, Jacobsen RH et al: Roentgenographic analysis of patellofemoral congruence. J Bone Joint Surg, 56A:1391, 1974 23. Bindelglass DE Cohen JL, Dorr LD: Patellar tilt and subluxation in total knee arthroplasty: relationship to pain, fixation and design. Clin Orthop 286:103, 1993 24. Petersilge WJ, Oishi CS, Kaufman KR et al: The effect of trochlear design on patellofemoral shear and compressive forces in total knee arthroplasty. Clin Orthop 309:124, 1994 25. Kayler DE, Lyttle D: Surgical interruption of patellar blood supply by total knee arthroplasty. Clin Orthop 229:22i, 1988 26. Scott RD, Turoff N, Ewald FC: Stress fracture of the patella following duopatellar total knee arthroplasty with patellar resurfacing. Clin Orthop 170: 147, 1982 27. Wetzner SM, Bezreh JS, Scott RD et al: Bone scanning in the assessment of patellar viability following knee replacement. Clin Orthop 199:215, 1985 28. McMahon MS, Scuderi GR, Glashow JL et al. Scintigraphic determination of patellar viability after excision of infrapatellar fat pad and/or lateral retinacular release in total knee arthroplasty. Clin Orthop 250:10, i990
M e d i a l i z a t i o n of t h e Patella in Total K n e e A r t h r o p l a s t y Kris Lewonowski, MD,* Lawrence D. Dorr, MD,J- Edward J. McPherson, MD,JGlen Huber, J- and Zhinian Wan, MD]-
Abstract: Patellar complications of total knee arthroplasty remain the most common
cause of pain and reoperation. Laboratory studies have suggested that medialization of the patella will improve tracking of the patella on the trochlea of the femoral component. The purpose of this study was to determine if clinical medialization of the patellar component on the patellar bone would improve tracking of the patella as demonstrated radiographically. Sixty-two knees were randomized so that 31 knees had a centrally placed patellar component and 31 had the patellar component placed on the medial two thirds of the patellar bone. There was no difference between the two groups with respect to either clinical or radiographic results in the first year after surgery. There was no improvement compared with previous reports in the incidence of tilt and displacement. The one improvement was a reduction in the incidence of lateral release. Thus, consequences of lateral release such as postoperative morbidity, avascular necrosis of the patella, and stress fracture of the patella can be avoided. It is recommended that the patellar component be placed on the medial two thirds of the patella to reduce the occurrence of lateral release. Tracking of the patella during surgery can be assessed using a single suture placed at the superior pole of the patella, and this technique in combination with the no-thumbs test provides an additional means of evaluation for patellar tracking. Key words: total knee arthroplasty, medialization, patella, lateral release.
The most c o m m o n cause of pain following total knee arthroplasty (TKA) is extensor m e c h a n i s m imbalance and poor tracking of the patella [1-4]. M a n y factors influence patellar tracking, including prosthetic design and surgical technique [5-10]. Resurfacing of the patella has b e e n s h o w n to significantly i m p r o v e pain relief obtained with TKA [2,3,11]; however, resurfacing of the patella increases the risk for complications, with subluxation of the patella being the most c o m m o n [3,4, 11,12]. In an effort to i m p r o v e patellar tracking m a n y authors have a d v o c a t e d the use of lateral
retinacular release [4,8,12-15]. Unfortunately, lateral retinacular release c a n n o t be p e r f o r m e d w i t h o u t complications, w h i c h include avascular necrosis, stress fracture, w o u n d slough, prol o n g e d w o u n d healing, a n d an increased incidence of infection [2,4,16,17]. Medialization of the patellar c o m p o n e n t on the patellar b o n e has also b e e n suggested as a m e a n s of i m p r o v i n g patellar tracking [ 5 , I 0 , 1 3 , 1 4 , 1 8 ] ; h o w e v e r , the only studies of the direct effect of a medial position of the patellar c o m p o n e n t o n patellar tracking a n d lateral retinacular t e n s i o n were perf o r m e d on cadaveric specimens [ 1 0 , I 9 ] . The h y p o t h e s i s of this study is that in TKA, medialization of the patellar c o m p o n e n t as c o m p a r e d with a central position i m p r o v e s postoperative patellar tilt, displacement, a n d the n e e d for lateral release.
From the *Kansas Orthopedic Center, Wichita, Kansas, and tUSC Center for Arthritis and Joint Implant Surgery, Los Angeles, California. Reprint requests: Lawrence D. Doff, MD; USC Center for Arthritis and Joint Implant Surgery, 1500 San Pablo Street, 7th Floor, Los Angeles, CA 90033.
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Materials and Methods B e t w e e n October I993 and July 1994, at the University of S o u t h e r n California Center for Arthritis and Joint Implant Surgery, primary TKA was performed in 74 knees in 74 patients. Seven patients were not included in this study because of the use of a prosthetic design different from that used for this study. Sixty-seven knees were assigned to have either a central or medial position of the patellar c o m p o n e n t d e p e n d e n t on w h e t h e r their hospital record n u m b e r was odd or even. Five knees that were assigned a medial position of the c o m p o n e n t had patellas that during surgery were f o u n d to be too small for this technique and were therefore excluded from the statistical comparison w h i c h was accomplished b e t w e e n the remaining 62 knees. Thirty-one knees!lhad a central patella and 31 a medial position o~ the patellar component. Demographics are listed in Table 1. The average age of the entire group was 67 years. All knees were operated with the Apollo cruciateretaining TKA system (Intermedics Orthopedics, Austin, TX). The Apollo TKA is a symmetric condylar design (Fig. 1). The femoral c o m p o n e n t with a symmetric trochlea is manufactured from cobalt-chrome metal. The tibia has a congruent dished articulation surface and was implanted either as an all-polyethylene tibia or as a modular insert in a stemmed cobalt-chrome component. The prosthetic patella is manufactured from ultra highmolecular-weight polyethylene. The shape is an
Fig. 1. (A) The Apollo cruciate-retaining knee is pictured here with the all-polyethylene tibial component. In the knees in this study, both the all-polyethylene tibial component and the metal-backed tibial component were used. (B) The trochlea is a symmetric design with a deepened patellar groove. The patella is an elliptical dome. There is a congruent fit of patella and trochlea.
Table 1. Demographics Categories
Central
Iganees 31 Women 19 Age (y} 66(39-80) Weight (lb) 186 Osteoarthritis 29 Rheumatoid arthritis 2 Follow-up period (too) 12
Medial 31 19 67(34-82) 173 28 3 12.2
P Value 1.00" 1.00" 1.00~ 0.73t 0.64" 0.75t
*Chi-square test. ~-t-test.
elliptical dome for all sizes but the smallest, which is a r o u n d dome shape. Four sizes of patellar component were available: 29 x 29 mm, 29 x 32 mm, 32 x 35 mm, and 35 x 38 m m w h i c h were oval dome shaped. Four sizes of the patellar c o m p o n e n t were used in this study: one 29 x 29 mm, thirty 29 x 32 mm, twenty-four 32 x 35 mm, and seven 35 × 38 mm. The three smallest components are 7 m m thick at the dome and 3.5 m m thick at the edge. The 35 x 3 8 - m m size is 8.5 m m thick at the dome. All operations but one were performed by one surgeon (L.D.D.); roentgenographic m e a s u r e m e n t s were performed by two i n d e p e n d e n t observers. Data were collected by two observers i n d e p e n d e n t of the surgeons, one of w h o m had participated in the radiographic measurements. The technique at surgery included the use of a t o u r n i q u e t unless the patient had diabetes or there were absent pulses in the popliteal fossa or distally. A medial parapatellar a r t h r o t o m y was used in all
Medialization of Patella inTKA
knees which e x t e n d e d from the top of the quadriceps t e n d o n to the medial aspect of the tibial tubercle. The quadriceps t e n d o n was incised so that it remained almost completely intact, with a 5 - m m strip of medial t e n d o n left attached to the vastus medialis muscle for t e n d o n repair at closure. The infrapatellar fat pad was partially excised to allow adequate exposure of the lateral tibial plateau. The patella was everted laterally and held by the use of a Kocher clamp placed on the quadriceps t e n d o n superiorly and the patellar ligament inferiorly. The patella was cut w i t h o u t the use of a jig from the medial to the lateral subchondral bone. The thickness of the patella was judged by palpation to be symmetric with equal bone medial, lateral, superior, and inferior. The level of the cut was n e v e r below the insertion of the quadriceps t e n d o n or patellar ligament. In the central patella three holes were drilled t h r o u g h a template placed centrally and a patellar c o m p o n e n t was selected that w o u l d resurface most of the patellar bone and result in a patellar height of 20 to 25 ram. In the medial components the drill holes were placed medially on the cut surface of the patella and a patellar c o m p o n e n t was selected that w o u l d leave one third of the lateral patellar bone uncovered. If the patient bone was too small to permit only two-thirds coverage with the smallest c o m p o n e n t , the patella was excluded from the study group. Intramedullary instrumentation was used for the f e m u r and tibia. The desired valgus position of the femoral compon e n t was 4 ° to 6 ° . The femoral c o m p o n e n t was rotationally aligned with the apex of the lateral epicondyle which routinely required external rotation of 3 °. The sizing of the femoral c o m p o n e n t did not usually permit it to be placed laterally. Tibial rotation was routinely internal, with the plastic eminence aligned with the medial third of the tibial tubercle. Fixation in each knee was accomplished by the use of b o n e - c e m e n t for the femoral, tibial, and patellar components. Patellar tracking was evaluated with the tourniquet inflated, and lateral release, w h e n necessary, was p e r f o r m e d with the t o u r n i q u e t inflated. Patellar tracking was confirmed with the t o u r n i q u e t deflated, immediately after c e m e n t fixation was completed. The decision to perform a lateral retinacular release was based on the tracking of the patellar c o m p o n e n t using a single-suture technique. This technique is a modification of the not h u m b s test described by Scott [I8,20]. With the n o - t h u m b s test, the patefla should track centrally in the trochlear groove w i t h o u t any m a n u a l support. With the single-suture technique, tilt or subluxation that occurs with the n o - t h u m b s test is
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eliminated w h e n a single suture of zero strength is placed to reattach the quadriceps t e n d o n at the superior pole of the patella. Contact of the patella had to be complete t h r o u g h o u t flexion. If this single suture breaks during flexion of the knee to 130 °, t h e n the extensor m e c h a n i s m is too tight and lateral release is necessary. If the suture does not break t h r o u g h full flexion of the knee, t h e n a lateral release is not necessary. If a lateral retinacular release was performed, no attempt was made to isolate and protect the superior-lateral geniculate artery. The lateral release was p e r f o r m e d by an incision from the tibial bone obliquely to the superior pole of the patella. This means that the entire lateral retinacular incision is distal to the lateral geniculate artery and does not require exposure of the lateral geniculate artery. No attempt was made to isolate and protect the superior-lateral geniculate artery during this release. The capsule was closed with interrupted sutures with the knee in extension. No intra-articular drains were placed in any of the 62 TKAs included in this study. All patients had epidural anesthesia and some also had general anesthesia. An indwelling epidural catheter was used for m o r p h i n e administration for 48 hours after surgery. In the recovery room, all patients had their operated extremity placed in a continuous passive m o t i o n machine with a range from full extension to 90 ° of flexion. The continuous passive m o t i o n machine was used only during a I 2 - h o u r period from 7:00 a.m. to 7:00 p.m. w h e n the patient was in bed. Several hours each day, the patient was out of bed participating in physical t h e r a p y or undergoing i n d e p e n d e n t activity. Some patients could not tolerate flexion to 90 ° and the range of passive m o t i o n was decreased to a comfortable level and gradually increased to 90 ° degrees over the succeeding 2 to 3 days. The continuous passive m o t i o n machine was continued until the patient had obtained active flexion of 80 ° for 24 hours or for a m a x i m u m of 5 days. Patients were discharged from the acute ward w h e n they had achieved i n d e p e n d e n c e in transfer and ambulation and achieved active range of m o t i o n of at least 15 ° to 80 °. Patients w h o had not achieved this level of function or w h o were not confident in their ability to live i n d e p e n d e n t l y at h o m e by the fifth postoperative day were transferred to the rehabilitation unit. After surgery, patients were evaluated at 6 weeks, 3 months, 6 months, and 1 year. At each visit, the range of m o t i o n of the knee was determined; this included the presence of extensor lag or flexion contracture and active extension and flexion. Before surgery and 6 m o n t h s afterward,
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patients w e r e evaluated by The Knee Society clinical rating system [21]. Before surgery and at each postoperative visit, roentgenographic examination of the knee included weight-bearing anteroposterior a n d lateral roentgenographs, as well as a lateral radiograph at 30 ° to 45 ° of flexion. The patella was further evaluated with a 45 ° M e r c h a n t ' s or sunrise view of the k n e e [22] (Fig. 2). The patellar view was p e r f o r m e d with a standard f r a m e to hold the k n e e flexed 45 ° and to support the cassette at the patient's ankle. The radiographic b e a m is directed at 60 ° to be tangential to the patella. Patellar tilt and displacement w e r e m e a s u r e d by the m e t h o d s described by Bindelglass et al. [23]. Preoperative patellar tilt was defined as the angle b e t w e e n a line f r o m the anterior limits of the femoral condyles and the equatorial line of the patella. After surgery, a line d r a w n t h r o u g h the patellar b o n e - p r o s t h e s i s interface was used to replace the equatorial patellar line. Patellar disp l a c e m e n t was m e a s u r e d as the distance b e t w e e n the center of the patella a n d a line d r a w n t h r o u g h the center of the femoral condyles. Final tilt and displacement w e r e m e a s u r e d f r o m a tangential pateflar radiograph obtained at the 6 - m o n t h postoperative visit (Fig. 3).
Fig. 3. The patella shows displacement of 8 ram. The angle is 5° so that it is not an abnormal tilt. The line drawing shows the technique used to measure the tilt and displacement of the patella. This patella has a medialized patellar component.
All data w e r e analyzed statistically using SYSTAT software (SPSS, Chicago, IL). The chi-square test was used for c o m p a r i s o n of sex, patellar tilt, a n d lateral release. Student's t-test was used for comparison of Knee Society scores, range of motion, and a n a t o m i c axis.
Results Sixty-two knees were included in this study; there were 19 w o m e n and 12 m e n in each group. Thirtyone knees had a central patella and 31 a medialized patella. The average age of the 62 patients was 67 years (range, 34--82 years). The duration of postoperative evaluation averaged 12 m o n t h s (range 6-15 months). Demographics for the two groups are listed in Table 1. B e t w e e n the central and medial groups there was not a n y statistical difference for sex, age, weight, diagnosis, side of the operated extremity, or length of follow-up period. The average p r e o p e r a t i v e Knee Society k n e e score for the central group was 46 and at last foll o w - u p visit was 87. For the medial group the average p r e o p e r a t i v e Knee Society k n e e score was 48 and at last follow-up visit was 90. Range of m o t i o n data are listed in Table 2. The distribution of the p r e o p e r a t i v e angulation of the knees is given in Table 3. After surgery, the
Table 2. Range of Motion
Preoperative 3 Months Fig. 2. The Merchant's view is shown with a patella that is centralized after surgery. The radiographs have been computer-enhanced to facilitate the bony outline.
Central Medial
110+15 110-+16
102_+14 101-+12
6 Months
Final
110_+14 115_+14 111-+ 11 117_+11
Medialization of Patella in TKA Table
3. Preoperative Angulation CO”
Central Medial
13 13
o”-10”
>loO
14 15
4 3
average anatomic axis for the central group was 4.6” (range, -2” to ll’), and that for the medial group was 4.1” (range, -3” to 13”). There was no statistical difference between the anatomic are of the central and medial groups before surgery, after surgery, or at the last follow-up examination. The average thickness of the patellar cut was 7.9 mm in the central group and 8.1 mm in the medial group (P = .61). There was no difference in patellar thickness because the patellar component thickness was constant at 7 mm in all but the 7 knees that had the 35 x 38-mm size. The patellar cut was symmetric in 22 of 31 in the central group and 23 of 31 in the medialized group (P = .78). Preoperative patellar tilt was observed in 10 knees of the central group and 12 knees of the medial group. After surgery, 4 of the 10 in the central group and 5 of the 12 in the medial group still demonstrated patellar tilt (Fig. 4). The average tilt angle for the central group decreased from 10.6” before to 8.3” after surgery, and the average tilt angle for the medial group decreased from 12” to 1 lo. Overall, of the 62 knees, 22 had postoperative tilt. Twelve knees that did not have preoperative tilt had postoperative tilt. Fifteen of the 22 knees had lateral tilt and 7 had medial tilt. Preoperative
Fig. 4. The top radiograph shows a knee with preoperative tilt. The bottom view shows the postoperative radiograph with the same tilt. There is statistical correlation to tilt after surgery when it is present before surgery.
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patellar displacement was observed in 5 knees in the central group and 10 knees in the medial group. After surgery, all 5 of the central group and 6 of the 10 in the medial group still demonstrated displacement (P = .74). In the central group the average displacement increased from 6.7 to 9.3 mm and, in the medial group, from 8.9 to 9.8 mm. Overall, 37 of 62 knees had postoperative displacement of at least 5 mm. Twenty-five knees that did not have preoperative displacement had postoperative displacement. Preoperative patellar tilt and displacement were observed in 4 knees in the central group and 6 knees in the medial group. After surgery, this increased to 7 knees in the central group and 9 knees in the medial group. Lateral release was required for satisfactory intraoperative tracking of the patella in 15 of 3 1 knees in the central group and in 4 of 31 knees in the medial group (P < .002). Of the 5 knees with the patella too small for medialization, 3 had a lateral release. In one patient in the medial group the lateral uncovered portion of the patella fractured while the patient was playing golf 4 months after surgery. A nondisplaced vertical fracture subsequently healed and the patient became clinically asymptomalic with a 12-month Knee Society score of 90. This patient did not have a lateral release at the time of surgery. Discussion The hypothesis in this study was that medial placement of the patellar component covering only the medial two thirds of the patellar bone would improve postoperative patellar tilt, displacement, and the need for lateral release. Medial placement of the patellar component in the laboratory resulted in statistically reduced lateral tracking and subluxation compared with a central position [lo]. Yoshii et al, also suggested that medial position of the patellar component decreased the Q-angle and decreased the lateral shear force on the patella [lo]. Patellar tilt and displacement from the patellofemoral groove occur commonly on radiographs of clinically satisfactory TICAs [9,23]. Bindelglass et al. reported incidences of 31.2% patellar tilt and 14.5% patellar displacement in clinically successful TICAs [23]. In that study, there was no change in patellar position 3 months after surgery. Therefore, the results measured in this study would not change with longer follow-up evaluation. In this study, we did not prove the hypothesis that medial displacement of the patellar component to cover only the medial two thirds of the cut patellar bone would decrease the incidence of patellar tilt
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and displacement. Forty-three percent of patients with preoperative tilt and 73% of patients with preoperative displacement continued to have the same tilt and displacement after surgery. This finding is in agreement with that of BindeIglass et al., w h o found a statistical correlation to the incidence of preoperative and postoperative tilt [23]. As was reported by Bindelglass et al., there is no correlation to the radiographic findings of tilt and displacement and clinical symptoms reported by the patients [23]. Although satisfactory patellar tracking was achieved in each of these patients during surgery, this intraoperative balance is only static balance. After surgery, the forces on the patella are both dynamic and static, which explains the inability during surgery to predict postoperative tilt and displacem e n t except in patients w h o exhibit this type of patellar tracking before surgery. The trochlea of the Apollo knee design used in these patients is symmetric. Both laboratory and clinical results have s h o w n n o difference with a symmetric or asymmetric trochlea in the tracking of the patella. In the study by Bindelglass et al., there was n o difference b e t w e e n the symmetric and asymmetric trochleas [23]. In a laboratory study by Petersilge et al., there was n o significant difference b e t w e e n the two shapes [24]. The patellar design used was an oval d o m e (Fig. 1). Perhaps a patella that was eccentric in shape so that it was thicker medially and t h i n n e r laterally w o u l d be as effective as medial displacement of this oval dome. Briard and H u n g e r f o r d reported that this eccentric-shaped patella p e r f o r m e d well in a clinical series [18]. C o n g r u e n c y of the patella to the trochlear groove w o u l d seem to be important. Creep of the p o l y e t h y l e n e of the patella does occur to p r o m o t e better tracking [9] so that the absolute importance of c o n g r u e n c y has not b e e n demonstrated. The hypothesis that the incidence of lateral release would be reduced with medial displacem e n t of the patellar c o m p o n e n t was proved. Lateral release was required in 48% of the central group and 13% of the medial group (P = .002). Lateral retinacular release is a c o m m o n technique employed to improve patellar tracking at the time of surgery. The question still u n a n s w e r e d is w h a t would be the incidence of patellar tilt and subluxation if no lateral releases were performed. In this study, we used the single-suture technique to determine satisfactory intraoperative patellar tracking. The single-suture technique is less severe t h a n the n o - t h u m b s test, which has b e e n c o m m o n l y used. With the single-suture technique, we still p e r f o r m e d a lateral retinacular release in 19 (31%)
knees. The incidence of tilt and displacement is not i m p r o v e d in knees that had lateral release as c o m p a r e d with those that did not [23]; however, Ritter and Campbell reported that lateral release p r e v e n t e d patellar tilt recognizable at surgery and c o n t i n u e d to provide postoperative patellar stability superior to that of those knees that had no release [t6]. Lateral retinacular release is assodated with complications which include avascular necrosis and stress fractures [3,I7,25-27]. Wetzner et al. found that lateral retinacular release resulted in decreased radionuclide uptake after surgery and stated the fat pad was important in preserving the blood supply to the patella {27]. M c M a h o n et al. used a scintigraphic technique to measure a higher incidence of vascular compromise of the patella after a lateral retinacular release and reported that excision of the infrapatellar fat pad did not compromise patellar vascularity [28]. In this study, the single advantage of medial displacement of the patellar c o m p o n e n t on the cut patellar bone is the reduction and r e q u i r e m e n t of lateral retinacular release for satisfactory intraoperative patellar tracking. Avoidance of lateral retinacular release does reduce the incidence of avascular necrosis and stress fractures of the patella and does reduce the postoperative morbidity of lateral knee ecchymosis and swelling and pain. Medial displacem e n t of the patellar c o m p o n e n t is not always possible because of the size of the patellar bone. In this study, in five knees the patellar c o m p o n e n t could not be displaced medially because the patellar bone was too small to permit this technique. The results in this study would suggest that satisfactory clinical tracking of the patella can be obtained with external rotation of the femoral c o m p o n e n t , alignment of the eminence of the tibia with the medial third of the tibial tubercle, and medial displacement of the patellar c o m p o n e n t to cover the medial two thirds of the cut patellar bone. Radiographic measurements of patellar position were not affected by the medial displacement of the patellar component. The single-suture technique for assessment of intraoperative patellar tracking was successful and allows an additional criterion to the not h u m b s test for evaluation of patellar stability.
References 1. Dorr LD, Boidaro RA: Technical considerations in total knee arthroplasty. Clin Orthop 205:5, 1986 2. Boyd AD, Ewald FC, Thomas WH et al: Long-term complications after total knee arthroplasty with or without resurfacing of the patella. J Bone Joint Surg 75A:674, I993
Medialization of Patella inTKA
3. Clayton ML, Thirupathi R: Patellar complications after total condylar arthroplasty. Clin Orthop 170:152, 1982 4. Rand JA: Current concepts review: the patellofemoral joint in total knee arthroplasty. J Bone Joint Surg 76A:612, 1994 5. Figgie I-IE, Goldberg YM, Heiple KH et al. The influence of tibial-patellofemoral location on function of the knee in patients with the posterior stabilized condylar knee prosthesis. J Bone Joint Surg 68A:1035, 1986 6. Grace JN, Rand JA: Patellar instability after total knee arthroplasty. Clin Orthop 237:184, 1988 7. Lotke PA, Echer ML: Influence of positioning of prosthesis in total knee replacement. J Bone Joint Surg 59A:77, 1977 8. Merkow RL, Soudry M, Insall JN: Patellar dislocation following total knee replacement. J Bone Joint Surg 67A:1321, 1985 9. Ranawat CS: The patello-femoral joint in total condylar knee arthroplasty: pros and cons based on five to ten year follow up observations. Clin Orthop 205:93, 1986 10. Yoshii I, Whiteside LA, Anouchi YS: The effect of patellar button placement and femoral component design on patellar tracking in total knee arthroplasty. Clin Orthop 275:211, 1992 11. InsalI J, Scott WN, Ranawat CS: The total condylar knee prosthesis: a report of two h u n d r e d and twenty cases. J Bone Joint Surg 61A:173, 1979 12. Johnson DE Eastwood DM: Lateral patellar release in knee arthroplasty: effect on w o u n d healing. J Arthroplasty 7 (suppl):427, 1992 13. Brick GW, Scott RD: The patellofemoral component of total knee arthroplasty. Clin Orthop 231:163, 1988 14. Insall JN, Haas SB: Complications of total knee arthroplasty, p. 905. In Insall JN (ed): Surgery of the knee. 2nd ed. Churchill Livingstone, New York, 1993 15. Ritter MA, Keating ME, Faris PM: Clinical, roentgenographic, and scintigraphic results after interruption of the superior lateral genicular artery during total knee arthroplasty. Clin Orthop 248:145, 1989
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16. Ritter MA, Campbell ED: Postoperative patellar complications with or without lateral release during total knee arthroplasty. Clin Orthop 219:163, 1987 17. Scuderi G, Scharf SC, Meltzer LP, Scott WN: The relationship of lateral releases to patella viability in total knee arthroplasty. J Arthroplasty 2:209, 1987 18. Briard JL, Hungerford DS: Patellofemoral instability in total knee arthroplasty. J Arthroplasty 4(suppl): 87, 1989 19. Bayley JC, Scott RD: Further observations on metalbacked patellar component failure. Clin Orthop 236:82, 1988 20. Scott RD: Prosthetic replacement of the patellofemoral joint. Orthop Clin North Am 10:129, 1979 21. Insall JN, Dorr LD, Scott RD, Scott WN: Rationale of The Knee Society clinical rating system. Clin Orthop 248:13, 1989 22. Merchant AC, Mercer RL, Jacobsen RH et al: Roentgenographic analysis of patellofemoral congruence. J Bone Joint Surg, 56A:1391, 1974 23. Bindelglass DE Cohen JL, Dorr LD: Patellar tilt and subluxation in total knee arthroplasty: relationship to pain, fixation and design. Clin Orthop 286:103, 1993 24. Petersilge WJ, Oishi CS, Kaufman KR et al: The effect of trochlear design on patellofemoral shear and compressive forces in total knee arthroplasty. Clin Orthop 309:124, 1994 25. Kayler DE, Lyttle D: Surgical interruption of patellar blood supply by total knee arthroplasty. Clin Orthop 229:22i, 1988 26. Scott RD, Turoff N, Ewald FC: Stress fracture of the patella following duopatellar total knee arthroplasty with patellar resurfacing. Clin Orthop 170: 147, 1982 27. Wetzner SM, Bezreh JS, Scott RD et al: Bone scanning in the assessment of patellar viability following knee replacement. Clin Orthop 199:215, 1985 28. McMahon MS, Scuderi GR, Glashow JL et al. Scintigraphic determination of patellar viability after excision of infrapatellar fat pad and/or lateral retinacular release in total knee arthroplasty. Clin Orthop 250:10, i990