- Email: [email protected]
Patellar tracking and flexion space balancing using a femoral component with asymmetrical femoral condyles
The Knee 6 Ž1999. 87᎐93
Patellar tracking and flexion space balancing using a femoral component with asymmetrical femoral condyles Richard S. Laskin The Hospital for Special Surgery, Cornell Uni¨ ersity Medical School, New York, NY, USA Received 26 July 1998; received in revised form 10 October 1998; accepted 26 October 1998
Abstract Using a neutrally positioned femoral component with asymmetrical posterior condyles ŽGenesis II., we were able to balance the flexion space without having to externally rotate the implant in the total knee patient. This avoided the problems with external rotation including incongruency in extension, femoral notching and trochlear overhang. Three percent of the patients required a lateral release. Patellar shift and patellar tracking were proper in over 95% of cases. The incidence of anterior knee pain was only 2%. The lateral release rate and the residual anterior knee pain we observed are lower than those reported in other published literature. 䊚 1999 Elsevier Science B.V. All rights reserved. Keywords: Lateral release; Patella; Total knee arthroplasty
1. Introduction External rotation of the femoral component during total knee arthroplasty has been used in order to balance the flexion space and enhance patellar tracking. This rotation of the femoral component, however, has potential deleterious effects on femoral᎐tibial tracking, femoral bone preparation, and patellar tracking in flexion. This study was undertaken to see whether flexion space balancing and proper patellar tracking could be achieved without femoral component external rotation and its attendant problems, by using a prosthesis with asymmetrical posterior femoral condyles and an anatomical trochlear groove ŽGenesis II..1 2. Materials and methods Beginning in July 1996, 100 consecutive primary total knee replacements performed by the senior au-
1
Smith and Nephew Orthopaedics, Memphis, TN.
thor were enrolled in the study. The indication for surgery in all these patients was advanced degenerative osteoarthritis. The operations were performed under hypotensive epidural anesthesia, in a vertical laminar flow enclosure. A midline skin incision and a median parapatellar capsular incision were used. The knee was exposed in a routine manner by subperiosteal dissection of the soft tissues inserting on the medial tibial metaphysis, excision of a portion of the fat pad, and sectioning of the epicondylar patellar ligament. The distal femur was resected using an intramedullary alignment instrument and planning for 5⬚ valgus angulation to the anatomical axis in the coronal plane and a 90⬚ angulation to the anatomical axis in the sagittal plane. An intramedullary tibial alignment instrument was likewise used for a planned tibial resection of 90⬚ in the coronal plane and 97⬚ in the sagittal plane. Anterior referencing instruments were used to guide the anterior, posterior, and chamfer resection of the femur. The accuracy of positioning of the cutting block was checked against the trans-epicondlar and
0968-0160r99r$ - see front matter 䊚 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 6 8 - 0 1 6 0 Ž 9 8 . 0 0 0 3 0 - 1
88
R.S. Laskin r The Knee 6 (1999) 87᎐93
Fig. 1. The position of the alignment jig is checked against the mid-trochlear and trans-epicondylar axes.
mid-trochlear axes ŽFig. 1.. The cutting block placement resulted in equal resections of equal posterior medial and lateral femoral condylar bone. The femoral components used in this study had asymmetrical posterior femoral condyles, lateral longer than medial ŽFig. 2a., subtending a 3⬚ arc of rotation around the neutral center point of the pros-
thesis ŽFig. 2b.. The trochlear groove was funneled laterally in its most proximal extent, and then extended distally in the midline. The femoral components used when the posterior cruciate ligament was sacrificed had a central box and cam for articulation with the central polyethylene post of the tibial component.
Fig. 2. Ža. Femoral component with asymmetrical posterior condyles ŽPCL retaining on left and posterior stabilized on the right.. Žb. Asymmetric femoral condyles fill asymmetric flexion space.
R.S. Laskin r The Knee 6 (1999) 87᎐93
89
Fig. 3. A 74-year-old female with osteoarthritis, 3 months after total knee replacement. Ža. Tunnel view in 90⬚ degrees of flexion with distraction to the tibia to demonstrate medial and lateral flexion space balancing using Genesis II prosthesis. Žb. Anteroposterior X-ray of the same patient demonstrating proper component position and alignment in extension.
The tibial component was modular. The asymmetrical metal base plate had a central fixation stem with two lateral fins. The polyethylene insert was conforming to the femoral component and available in a variety of thicknesses. The patellar implants were symmetrical and biconvex. The implant was inset into the bone and centered on the patellar ridge, functionally medializing the implant. An amount of bone and cartilage equal in thickness to the thickness of the prosthetic component was removed so as not to overthicken the patella᎐patellar component composite. With the trial implants in place, the knees were tested for stability. Varus᎐valgus stability was ascertained both with the knee in full extension and with the knee slightly flexed to relax the posterior capsule. At 90⬚ of flexion, the knees were checked both for varus᎐valgus and antero-posterior stability. Finally, the knee was placed through a trial range of motion to assess implant-tracking, range of motion and limb alignment. If necessary changes in implant size, thickness, or further ligament balancing was performed. These parameters were again tested after the permanent implants were cemented in place. If the patella was found to be tilted or to sublux laterally, a lateral retinacular release was performed, with an attempt made to spare the lateral superior genicular artery.
The lateral release procedures, when required, were made from without inward to leave the synovial layer intact. Post-operatively all patients were started on continuous passive motion, 1᎐2 h after surgery Ž0᎐60⬚.. This was increased daily. Patient controlled analgesia via the epidural catheter was maintained for 48 h after surgery. The patients were allowed weight bearing to tolerance using biannual support on the day after surgery. They were allowed to use unilateral support when muscle strength had returned sufficiently Žon average the 5th day after surgery.. Peri-operative antibiotics were administered 20 min before the skin incision and continued for 24 h after surgery. All patients were placed on Coumadin as an anti-thromboembolic drug for 4 weeks. Foot pumps were used until the patients were fully ambulatory. Pre- and post-operative data were collected including subjective and objective clinical findings, radiographic findings and a SF-36 evaluation. Patients were evaluated pre-operatively, at 6 weeks, 3 months, and at the yearly anniversaries of their surgery. The X-rays taken included a weight-bearing antero-posterior view ŽFig. 3a., and a recumbent lateral view. The leg was next positioned with the tibia hanging vertically unsupported with the knee flexed 90⬚ so as to provide a distractive force to the knee, and a tunnel antero-posterior view taken with the
R.S. Laskin r The Knee 6 (1999) 87᎐93
90
Fig. 4. Trapezoidal flexion space results from 90⬚ resection of the proximal tibia and equal resection of the posterior femor Žleft.. This is improperly filled by a symmetrical implant resulting in flexion space instability deformity Žright..
Fig. 5. Measurement for patellar tilt Žleft. and patellar shift Žright..
X-ray beam tangential to the tibial component ŽFig. 3b.. Finally a Merchant patella view was taken ŽFig. 5.. Statistical analysis of non-parametric data was performed using the 2 analysis, while analysis of parametric data was performed using the two-tailed Student’s t-test. In both tests, a value was felt to be significant if P- 0.05. X-rays were measured using the Ex-Caliper goniometer 2 . The accuracy of this electronic measuring device was 1 mm and 1.5⬚. Two separate independent observers were used and the
2
Eisenlohr Technologies, Davis, California.
mean taken of each of their two measurements. The demographic data for the patients in this study are noted in Table 1 in 54 knees, the posterior cruciate ligament was retained, while in the remaining 46, the ligament was sacrificed and a posterior stabilized implant used. Only one of the original 100 patients was lost at follow-up and none died. 3. Results Three patients required a lateral release during surgery to maintain proper patellar tracking. In two of these patients, the pre-operative femoral tibial alignment was q18⬚ and q15 of valgus degrees, respec-
R.S. Laskin r The Knee 6 (1999) 87᎐93 Table 1 Pre-operative demographic data 100 patient undergoing primary total knee replacement Diagnosis: osteoarthritis Age: 58᎐89 Žmean s 71. Weight: 48᎐99 kg Žmean s 89 kg. 64 Males, 36 Females Pre-Op american knee society knee score: 18᎐54 Žmean s 30. Pre-Op ROM: 94᎐125⬚ Žmean 114⬚. Pre-Op alignment Žfemoral-tibial angle. - q10⬚ 4 q10⬚ to q3⬚ 23 - q3 73 Able to walk stairs in reciprocal manner: 24 patients lateral patellar subluxation: 3 knees
tively and in both cases, the patella was subluxed pre-operatively. In the remaining case, the femoral tibial alignment was 7⬚ of valgus and the patella tracked normally pre-operatively. In all the three cases, it had been ascertained during surgery that there was neither improper internal rotation of the femoral or tibial components, nor overthickening of the patella. At 1 year post-operatively, patella tilt, as measured on the Merchant view, averaged 1.3⬚ with a range of 0᎐15⬚ laterally. There were no knees with a medial tilt. In 93 knees, there was no patella tilt ŽTable 2.. Likewise at 1 year after surgery, patellar displacement averaged 1.5 mm laterally with a range from 1 mm medially to 9 mm laterally. In 92 cases, there was no patellar displacement at all. There was no statistical difference in the group in which the PCL had been retained and that in which a posterior stabilized prosthesis had been used as related to patellar tilt Ž P) 0.1. or patellar displacement Ž P) 0.1.. Thirty-four of these knees have been evaluated 2 years after surgery. There was no statistical change noted in the position of the patella as related to tilt or displacement from that seen at the 1-year follow-up evaluation. At 1 year after surgery, 97 patients had neither anterior knee pain when ascending or descending Table 2 Patellar alignment post operatively Žmerchant tangential patellar X rays. Patellar tilt Žmean 1.3⬚. None - 5⬚ 5⬚᎐10⬚ ) 10⬚
93 knees 5 knees 1 knee 1 knee
Patellar displacement Žmean s 1.5 mm. None 1᎐5 mm laterally ) 5 mm laterally Medial
92 knees 7 knees 1 knee 0 knee
91
stairs, nor localized tenderness with patello-femoral compression. One patient had moderate scar tenderness over the patella, while two others complained of moderate pain with descending stairs. The 34 knees evaluated 2 years after surgery demonstrated that 32 patients had neither anterior knee pain nor localized anterior tenderness. One patient had fallen striking the knee anteriorly but had suffered no evidence of fracture or implant loosening on X-ray. One patient who had had no pain at all with ascending or descending stairs at 1 year after surgery complained of episodic discomfort descending stairs at 2 years after surgery. The mean pre-operative flexion contracture was 6⬚. The mean post-operative flexion contracture was 0.5⬚ with a range of 0᎐5⬚. The mean pre-operative passive flexion was 108⬚, while the mean flexion at 1 year after surgery was 116⬚ with a range from 85 to 135⬚. The eventual flexion range was statistically similar between the PCL retaining and PS knees Ž P- 0.1. The medial femoral᎐tibial and lateral femoral᎐ tibial ‘joint spaces’ were equal as measured on tunnel views in 91 knees. In five knees, it was 1 mm greater on the lateral side, in two, it was 1 mm greater on the medial side, and in one, it was 3 mm greater on the lateral side. There was no statistical difference in the measurements of the flexion space in the PCL sparing and posterior stabilized groups Ž P) 0.1.. 4. Discussion The normal tibial articular surface is oriented at 87᎐88⬚ to the anatomical axis of the tibia in its coronal plane. If, during total knee replacement surgery, the tibia is resected at this angulation and equal amounts of bone resected from the posterior medial and lateral femoral condyles, a rectangular flexion space is formed. If this space is filled with femoral and tibial components of equal thickness medially and laterally as to tense the capsular sleeve, the knee will be stable in flexion. At the present time, most surgeons resect the proximal tibia at 90⬚ to the anatomical axis in order to prevent an inadvertent varus over-resection. In this situation, if equal amounts of postero-medial and postero-lateral femoral condylar bone is removed, a trapezoidal flexion space is formed, larger laterally than medially ŽFig. 4 Žleft... If a femoral component of equal thickness medial and lateral is used, the flexion space is not balanced. It is either too loose laterally or tight medially ŽFig. 4 Žright... Previous studies have shown that this situation results in an increased incidence of medial tibial pain and tibial component loosening, as compared to situation in which the flexion spaced is balanced w5x. To overcome this, Insall w4x and Moreland w6,7x
92
R.S. Laskin r The Knee 6 (1999) 87᎐93
suggested external rotation of the femoral cutting blocks by 2᎐3⬚ to recreate a rectangular flexion space. As a secondary benefit of this femoral component external rotation, the proximal portion of the trochlear groove is displaced laterally, and this can assist in patellar capture as the knee begins flexing. The exact method of determining how to assess this external rotation has varied. Authors have recommended using the trans-epicondylar line w4x, the midtrochlear line w1x, the tibial shaft axis w10x, or some combination of all of these. This external rotation of the femoral component addresses the anatomical situation of a trapezoidal flexion space by a deliberate malpositioning the femoral component. Ries w9x, however, has shown that such external rotation of the femoral component, although it can result in flexion space balancing, can also lead to numerous secondary problems. If the tibial component is not externally rotated likewise, the knee will not be congruous in extension and this can lead to increased polyethylene wear. If the tibial component is externally rotated, however, it can overhang the plateau postero-laterally, impinging on the popliteus tendon causing pain and a palpable snap. Furthermore, this can result in a segment of the postero-medial tibial plateau being uncovered by the implant. External rotation of the femoral cutting block can also lead to lateral anterior femoral cortical notching and lifting off of the medial prosthetic trochlear surface from the anterior surface of the femur. A further potential problem with external rotation of the femoral component relates to the location of the prosthetic trochlear groove. An externally rotated component placed the proximal portion of the groove laterally and could aid in patellar tracking in the extended position of the knee. The most distal part of the groove, however, is placed medial to the midline, a distinctly non-anatomical position, and this can result in abnormal tracking and increased polyethylene wear. In theory this might be compensated for by placing the femoral component somewhat lateral on the femur, however, this is quickly limited by the size of the condyles. The difference in the posterior dimensions of the medial and lateral femoral condyles results in a femoral component which is trapezoidal posteriorly, and fills the trapezoidal flexion space without the necessity for non-anatomical rotation of the femoral component. The difference in the lengths of the two femoral condyles yields an ‘effective’ external rotation posteriorly of 3⬚ degrees. At the onset of this study, there was concern as to whether this ‘effective’ 3⬚ of external rotation was the proper amount and whether it varied between knees. Consequently, the position of the cutting block was compared anatomically to the mid trochlear and
trans-epicondylar references prior to pinning it in situ. Since the mid trochlear line and trans-epicondlar lines describe an externally rotated femoral component, the references on our guide were 3⬚ rotated from these landmarks. In all but three knees, the effective rotation obtained by the asymmetry of the posterior condyles was correct. The three cases in which it was not, were in patients with a pre-operative valgus deformity ) 11⬚ and a lateral femoral condyle that was dysplastic both distally and posteriorly. This dysplasia of the lateral femoral condyle is often seen in patients with a severe valgus deformity and may be the etiology of their lateral compartmental degenerative disease. Two of these three patients likewise had a laterally subluxed patella pre-operatively. The intrinsic ‘effective’ 3⬚ external rotation therefore was used as a starting point, which, for the majority of knees, was an end point as well. When the leg was positioned for the tunnel X-ray, the weight of the lower leg was used to apply a mild distractive force to the knee joint. We used this radiographic technique to supplement our clinical impression of varus᎐valgus stability when the knee was flexed. In 91 knees, the medial and lateral spaces were equal in size. The purpose of a lateral retinacular release is to allow the patella to track normally in the trochlear groove without tilt or subluxation. The incidence of lateral release procedures has varied widely in the literature w3,8x and this variation is based upon multiple factors. For example, the type of pre-operative deformity can influence the rate of lateral release procedures, being greatest in patients with a fixed valgus contracture and those with a pre-operative subluxation of the patella. The surgical incision has been implicated as well. Engh w2x described a 70% reduction in the requirement for lateral release procedures when he changed from a median parapatellar to a mid vastus capsular incision. Femoral component design has an obvious effect as well. Femoral components with a high conforming lateral trochlear ridge cause increased tension on the lateral retinaculum and increase the rate of lateral tilt and subluxation. Likewise, trochlear surfaces that are completely flat have resulted in a very high incidence of lateral subluxation and dislocation. Finally, patellar preparation has an effect on patellar tracking and the necessity for a lateral retinacular release. If insufficient patellar bone is removed, the composite bone and implant are thickened relative to the pre-operative state and this causes not only increased tension in the lateral retinaculum but increased contact stresses in the polyethylene as well. In the normal patella, the lateral facet is larger than the medial facet, so that the patellar ridge is medial to
R.S. Laskin r The Knee 6 (1999) 87᎐93
the mid-point of the bone. This should be reproduced in the prosthetic patella as well and is the rationale for medialization of the symmetrical patellar implant. In a patient with a patella that is much larger mediolaterally than it is antero-posteriorly, this may result in an uncovering of the patellar bone laterally which can come in contact with the femoral trochlear surface. There is no uniform opinion as to whether the most lateral portion of the lateral facet should be removed. Possibly, the patellar meniscus, which almost routinely covers the periphery of the patellar implant and the bone, may protect against impingement and anterior knee pain. Despite the lower lateral release rate noted in this study Ž3%., there was no compromise in the post-operative patellar tracking as measured by patellar tilt and patellar shift on the post-operative Merchant views. The mean lateral tilt was just over 1⬚ with over 84% of the knees having no tilt at all. Patellar tilt and or subluxation, when it does occur, is usually manifested within the first 6 months after the arthroplasty. For that reason, it is felt that this 1-year roentgenographic evaluation is sufficiently sensitive to evaluate this phenomenon. Likewise, although there have been no prior studies roentgenographically evaluating the flexion space post-operatively, it would seem that if there were instability due to the surgical procedure, itself too would be manifest by the period of time after surgery. In measuring patellar tilt, the orientation of the transverse axis of the patella, as described by Grelsamer et al. w10x was compared to a line joining the anterior medial and antero lateral edges of the femoral trochlear groove. A second measurement was made for each patient to determine the patellar implant tilt. In this case, we measured the angulation between the undersurface marker wire of the patella and the anterior condylar line. Both of these measurements are important, the former in ascertaining the overall tracking of the patella with its implant, and the latter in ascertaining the proper placement of the implant in the bone. We did not measure the patella angle as reference to the posterior femoral condyle nor did we elect to measure the congruence angle in these patients. Not only has Inoue shown that congruence angle measurements are not sensitive to measure patella subluxation, a reproducible method of measuring these angles in the presence of a knee implant could not be made. Anterior knee pain after surgery can have multiple etiologies. It is important when evaluating this pain to exclude those situations in which the lateral numbness, often seen for many months after an anterior midline incision, not to be confused with pain. The
93
commonest causes that we had found in the past for anterior knee pain were maltracking of the patella, overstuffing of the patello-femoral joint space and avascular necrosis of the patella. True anterior knee pain is most prominent with ascending or descending stairs or ramps, when arising from a chair, and when kneeling. Objectively it can be reproduced by direct pressure on the patella against the trochlear groove. The incidence of anterior knee pain in this study was only 2%. 5. Summary Using a neutrally positioned femoral component with asymmetrical posterior condyles, we were able to balance the flexion space without having to externally rotate the implant. Three percent of the patients required a lateral release. Patellar tracking and flexion space balancing were correct in the overwhelming percentage of patients, and the incidence of anterior knee pain was 2%. We feel that this surgical technique and implant has obviated the problems associated with external rotation of the femoral component which were seen when a symmetrical femoral component was used. The lateral release rate and the residual anterior knee pain we observed are lower than those reported in other published literature. References w1x Anouchi YS, Whiteside LA, Kaiser AD Milliano MT. The effects of axial rotation alignment of the femoral component on knee stability and patellar tracking in total knee arthroplasty as demonstrated on autopsy specimens. Clin Orthop 1995;287:170᎐177. w2x Engh GA, Parks NL, Ammeen DJ. Influence of surgical approach on lateral retinacular releases in total knee arthroplasty. Clin Orthop 1998;331:56᎐63. w3x Grace JN, Rand JA. Patellar instability after total knee arthroplasty. Clin Orthop 1988;237:184᎐189. w4x Insall JN, Windsor RE, Scott WN, Kelly MA, Aglieti P. Surgery of the knee. New York: Churchill Livingstone, 1993. w5x Laskin RS. Flexion space configuration in total knee replacement ŽAbstract.. J Arthroplasty 1995;10:657᎐661. w6x Moreland JR. Mechanisms of failure in total knee arthroplasty. Clin Orthop 1988;226:49᎐64. w7x Moreland JR, Bassett LW, Hanker GJ. Radiographic analysis of the axial alignment of the lower extremity. J Bone Joint Surg 1987;69A:745᎐749. w8x Rhoads DD, Noble PC, Reuber JD, Mahoney OM, Tullos HS. The effect of femoral component position on the kinematics of total knee arthroplasty. Clin Orthop 1998; 286:122᎐129. w9x Ries MD, Salehi A, Laskin RS, Bourne RB, Rand JA, Gustilo RB. Can rotational congruity be achieved in both flexion and extension when the femoral component is externally rotated in total knee arthroplasty ŽAbstract.. The Knee 1998;5:37᎐42. w10x Stiehl JB, Cherveny PM. Femoral rotational alignment using the tibial shaft axis in total knee arthroplasty. Clin Orthop 1996;331:47᎐55.
Patellar tracking and flexion space balancing using a femoral component with asymmetrical femoral condyles Richard S. Laskin The Hospital for Special Surgery, Cornell Uni¨ ersity Medical School, New York, NY, USA Received 26 July 1998; received in revised form 10 October 1998; accepted 26 October 1998
Abstract Using a neutrally positioned femoral component with asymmetrical posterior condyles ŽGenesis II., we were able to balance the flexion space without having to externally rotate the implant in the total knee patient. This avoided the problems with external rotation including incongruency in extension, femoral notching and trochlear overhang. Three percent of the patients required a lateral release. Patellar shift and patellar tracking were proper in over 95% of cases. The incidence of anterior knee pain was only 2%. The lateral release rate and the residual anterior knee pain we observed are lower than those reported in other published literature. 䊚 1999 Elsevier Science B.V. All rights reserved. Keywords: Lateral release; Patella; Total knee arthroplasty
1. Introduction External rotation of the femoral component during total knee arthroplasty has been used in order to balance the flexion space and enhance patellar tracking. This rotation of the femoral component, however, has potential deleterious effects on femoral᎐tibial tracking, femoral bone preparation, and patellar tracking in flexion. This study was undertaken to see whether flexion space balancing and proper patellar tracking could be achieved without femoral component external rotation and its attendant problems, by using a prosthesis with asymmetrical posterior femoral condyles and an anatomical trochlear groove ŽGenesis II..1 2. Materials and methods Beginning in July 1996, 100 consecutive primary total knee replacements performed by the senior au-
1
Smith and Nephew Orthopaedics, Memphis, TN.
thor were enrolled in the study. The indication for surgery in all these patients was advanced degenerative osteoarthritis. The operations were performed under hypotensive epidural anesthesia, in a vertical laminar flow enclosure. A midline skin incision and a median parapatellar capsular incision were used. The knee was exposed in a routine manner by subperiosteal dissection of the soft tissues inserting on the medial tibial metaphysis, excision of a portion of the fat pad, and sectioning of the epicondylar patellar ligament. The distal femur was resected using an intramedullary alignment instrument and planning for 5⬚ valgus angulation to the anatomical axis in the coronal plane and a 90⬚ angulation to the anatomical axis in the sagittal plane. An intramedullary tibial alignment instrument was likewise used for a planned tibial resection of 90⬚ in the coronal plane and 97⬚ in the sagittal plane. Anterior referencing instruments were used to guide the anterior, posterior, and chamfer resection of the femur. The accuracy of positioning of the cutting block was checked against the trans-epicondlar and
0968-0160r99r$ - see front matter 䊚 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 6 8 - 0 1 6 0 Ž 9 8 . 0 0 0 3 0 - 1
88
R.S. Laskin r The Knee 6 (1999) 87᎐93
Fig. 1. The position of the alignment jig is checked against the mid-trochlear and trans-epicondylar axes.
mid-trochlear axes ŽFig. 1.. The cutting block placement resulted in equal resections of equal posterior medial and lateral femoral condylar bone. The femoral components used in this study had asymmetrical posterior femoral condyles, lateral longer than medial ŽFig. 2a., subtending a 3⬚ arc of rotation around the neutral center point of the pros-
thesis ŽFig. 2b.. The trochlear groove was funneled laterally in its most proximal extent, and then extended distally in the midline. The femoral components used when the posterior cruciate ligament was sacrificed had a central box and cam for articulation with the central polyethylene post of the tibial component.
Fig. 2. Ža. Femoral component with asymmetrical posterior condyles ŽPCL retaining on left and posterior stabilized on the right.. Žb. Asymmetric femoral condyles fill asymmetric flexion space.
R.S. Laskin r The Knee 6 (1999) 87᎐93
89
Fig. 3. A 74-year-old female with osteoarthritis, 3 months after total knee replacement. Ža. Tunnel view in 90⬚ degrees of flexion with distraction to the tibia to demonstrate medial and lateral flexion space balancing using Genesis II prosthesis. Žb. Anteroposterior X-ray of the same patient demonstrating proper component position and alignment in extension.
The tibial component was modular. The asymmetrical metal base plate had a central fixation stem with two lateral fins. The polyethylene insert was conforming to the femoral component and available in a variety of thicknesses. The patellar implants were symmetrical and biconvex. The implant was inset into the bone and centered on the patellar ridge, functionally medializing the implant. An amount of bone and cartilage equal in thickness to the thickness of the prosthetic component was removed so as not to overthicken the patella᎐patellar component composite. With the trial implants in place, the knees were tested for stability. Varus᎐valgus stability was ascertained both with the knee in full extension and with the knee slightly flexed to relax the posterior capsule. At 90⬚ of flexion, the knees were checked both for varus᎐valgus and antero-posterior stability. Finally, the knee was placed through a trial range of motion to assess implant-tracking, range of motion and limb alignment. If necessary changes in implant size, thickness, or further ligament balancing was performed. These parameters were again tested after the permanent implants were cemented in place. If the patella was found to be tilted or to sublux laterally, a lateral retinacular release was performed, with an attempt made to spare the lateral superior genicular artery.
The lateral release procedures, when required, were made from without inward to leave the synovial layer intact. Post-operatively all patients were started on continuous passive motion, 1᎐2 h after surgery Ž0᎐60⬚.. This was increased daily. Patient controlled analgesia via the epidural catheter was maintained for 48 h after surgery. The patients were allowed weight bearing to tolerance using biannual support on the day after surgery. They were allowed to use unilateral support when muscle strength had returned sufficiently Žon average the 5th day after surgery.. Peri-operative antibiotics were administered 20 min before the skin incision and continued for 24 h after surgery. All patients were placed on Coumadin as an anti-thromboembolic drug for 4 weeks. Foot pumps were used until the patients were fully ambulatory. Pre- and post-operative data were collected including subjective and objective clinical findings, radiographic findings and a SF-36 evaluation. Patients were evaluated pre-operatively, at 6 weeks, 3 months, and at the yearly anniversaries of their surgery. The X-rays taken included a weight-bearing antero-posterior view ŽFig. 3a., and a recumbent lateral view. The leg was next positioned with the tibia hanging vertically unsupported with the knee flexed 90⬚ so as to provide a distractive force to the knee, and a tunnel antero-posterior view taken with the
R.S. Laskin r The Knee 6 (1999) 87᎐93
90
Fig. 4. Trapezoidal flexion space results from 90⬚ resection of the proximal tibia and equal resection of the posterior femor Žleft.. This is improperly filled by a symmetrical implant resulting in flexion space instability deformity Žright..
Fig. 5. Measurement for patellar tilt Žleft. and patellar shift Žright..
X-ray beam tangential to the tibial component ŽFig. 3b.. Finally a Merchant patella view was taken ŽFig. 5.. Statistical analysis of non-parametric data was performed using the 2 analysis, while analysis of parametric data was performed using the two-tailed Student’s t-test. In both tests, a value was felt to be significant if P- 0.05. X-rays were measured using the Ex-Caliper goniometer 2 . The accuracy of this electronic measuring device was 1 mm and 1.5⬚. Two separate independent observers were used and the
2
Eisenlohr Technologies, Davis, California.
mean taken of each of their two measurements. The demographic data for the patients in this study are noted in Table 1 in 54 knees, the posterior cruciate ligament was retained, while in the remaining 46, the ligament was sacrificed and a posterior stabilized implant used. Only one of the original 100 patients was lost at follow-up and none died. 3. Results Three patients required a lateral release during surgery to maintain proper patellar tracking. In two of these patients, the pre-operative femoral tibial alignment was q18⬚ and q15 of valgus degrees, respec-
R.S. Laskin r The Knee 6 (1999) 87᎐93 Table 1 Pre-operative demographic data 100 patient undergoing primary total knee replacement Diagnosis: osteoarthritis Age: 58᎐89 Žmean s 71. Weight: 48᎐99 kg Žmean s 89 kg. 64 Males, 36 Females Pre-Op american knee society knee score: 18᎐54 Žmean s 30. Pre-Op ROM: 94᎐125⬚ Žmean 114⬚. Pre-Op alignment Žfemoral-tibial angle. - q10⬚ 4 q10⬚ to q3⬚ 23 - q3 73 Able to walk stairs in reciprocal manner: 24 patients lateral patellar subluxation: 3 knees
tively and in both cases, the patella was subluxed pre-operatively. In the remaining case, the femoral tibial alignment was 7⬚ of valgus and the patella tracked normally pre-operatively. In all the three cases, it had been ascertained during surgery that there was neither improper internal rotation of the femoral or tibial components, nor overthickening of the patella. At 1 year post-operatively, patella tilt, as measured on the Merchant view, averaged 1.3⬚ with a range of 0᎐15⬚ laterally. There were no knees with a medial tilt. In 93 knees, there was no patella tilt ŽTable 2.. Likewise at 1 year after surgery, patellar displacement averaged 1.5 mm laterally with a range from 1 mm medially to 9 mm laterally. In 92 cases, there was no patellar displacement at all. There was no statistical difference in the group in which the PCL had been retained and that in which a posterior stabilized prosthesis had been used as related to patellar tilt Ž P) 0.1. or patellar displacement Ž P) 0.1.. Thirty-four of these knees have been evaluated 2 years after surgery. There was no statistical change noted in the position of the patella as related to tilt or displacement from that seen at the 1-year follow-up evaluation. At 1 year after surgery, 97 patients had neither anterior knee pain when ascending or descending Table 2 Patellar alignment post operatively Žmerchant tangential patellar X rays. Patellar tilt Žmean 1.3⬚. None - 5⬚ 5⬚᎐10⬚ ) 10⬚
93 knees 5 knees 1 knee 1 knee
Patellar displacement Žmean s 1.5 mm. None 1᎐5 mm laterally ) 5 mm laterally Medial
92 knees 7 knees 1 knee 0 knee
91
stairs, nor localized tenderness with patello-femoral compression. One patient had moderate scar tenderness over the patella, while two others complained of moderate pain with descending stairs. The 34 knees evaluated 2 years after surgery demonstrated that 32 patients had neither anterior knee pain nor localized anterior tenderness. One patient had fallen striking the knee anteriorly but had suffered no evidence of fracture or implant loosening on X-ray. One patient who had had no pain at all with ascending or descending stairs at 1 year after surgery complained of episodic discomfort descending stairs at 2 years after surgery. The mean pre-operative flexion contracture was 6⬚. The mean post-operative flexion contracture was 0.5⬚ with a range of 0᎐5⬚. The mean pre-operative passive flexion was 108⬚, while the mean flexion at 1 year after surgery was 116⬚ with a range from 85 to 135⬚. The eventual flexion range was statistically similar between the PCL retaining and PS knees Ž P- 0.1. The medial femoral᎐tibial and lateral femoral᎐ tibial ‘joint spaces’ were equal as measured on tunnel views in 91 knees. In five knees, it was 1 mm greater on the lateral side, in two, it was 1 mm greater on the medial side, and in one, it was 3 mm greater on the lateral side. There was no statistical difference in the measurements of the flexion space in the PCL sparing and posterior stabilized groups Ž P) 0.1.. 4. Discussion The normal tibial articular surface is oriented at 87᎐88⬚ to the anatomical axis of the tibia in its coronal plane. If, during total knee replacement surgery, the tibia is resected at this angulation and equal amounts of bone resected from the posterior medial and lateral femoral condyles, a rectangular flexion space is formed. If this space is filled with femoral and tibial components of equal thickness medially and laterally as to tense the capsular sleeve, the knee will be stable in flexion. At the present time, most surgeons resect the proximal tibia at 90⬚ to the anatomical axis in order to prevent an inadvertent varus over-resection. In this situation, if equal amounts of postero-medial and postero-lateral femoral condylar bone is removed, a trapezoidal flexion space is formed, larger laterally than medially ŽFig. 4 Žleft... If a femoral component of equal thickness medial and lateral is used, the flexion space is not balanced. It is either too loose laterally or tight medially ŽFig. 4 Žright... Previous studies have shown that this situation results in an increased incidence of medial tibial pain and tibial component loosening, as compared to situation in which the flexion spaced is balanced w5x. To overcome this, Insall w4x and Moreland w6,7x
92
R.S. Laskin r The Knee 6 (1999) 87᎐93
suggested external rotation of the femoral cutting blocks by 2᎐3⬚ to recreate a rectangular flexion space. As a secondary benefit of this femoral component external rotation, the proximal portion of the trochlear groove is displaced laterally, and this can assist in patellar capture as the knee begins flexing. The exact method of determining how to assess this external rotation has varied. Authors have recommended using the trans-epicondylar line w4x, the midtrochlear line w1x, the tibial shaft axis w10x, or some combination of all of these. This external rotation of the femoral component addresses the anatomical situation of a trapezoidal flexion space by a deliberate malpositioning the femoral component. Ries w9x, however, has shown that such external rotation of the femoral component, although it can result in flexion space balancing, can also lead to numerous secondary problems. If the tibial component is not externally rotated likewise, the knee will not be congruous in extension and this can lead to increased polyethylene wear. If the tibial component is externally rotated, however, it can overhang the plateau postero-laterally, impinging on the popliteus tendon causing pain and a palpable snap. Furthermore, this can result in a segment of the postero-medial tibial plateau being uncovered by the implant. External rotation of the femoral cutting block can also lead to lateral anterior femoral cortical notching and lifting off of the medial prosthetic trochlear surface from the anterior surface of the femur. A further potential problem with external rotation of the femoral component relates to the location of the prosthetic trochlear groove. An externally rotated component placed the proximal portion of the groove laterally and could aid in patellar tracking in the extended position of the knee. The most distal part of the groove, however, is placed medial to the midline, a distinctly non-anatomical position, and this can result in abnormal tracking and increased polyethylene wear. In theory this might be compensated for by placing the femoral component somewhat lateral on the femur, however, this is quickly limited by the size of the condyles. The difference in the posterior dimensions of the medial and lateral femoral condyles results in a femoral component which is trapezoidal posteriorly, and fills the trapezoidal flexion space without the necessity for non-anatomical rotation of the femoral component. The difference in the lengths of the two femoral condyles yields an ‘effective’ external rotation posteriorly of 3⬚ degrees. At the onset of this study, there was concern as to whether this ‘effective’ 3⬚ of external rotation was the proper amount and whether it varied between knees. Consequently, the position of the cutting block was compared anatomically to the mid trochlear and
trans-epicondylar references prior to pinning it in situ. Since the mid trochlear line and trans-epicondlar lines describe an externally rotated femoral component, the references on our guide were 3⬚ rotated from these landmarks. In all but three knees, the effective rotation obtained by the asymmetry of the posterior condyles was correct. The three cases in which it was not, were in patients with a pre-operative valgus deformity ) 11⬚ and a lateral femoral condyle that was dysplastic both distally and posteriorly. This dysplasia of the lateral femoral condyle is often seen in patients with a severe valgus deformity and may be the etiology of their lateral compartmental degenerative disease. Two of these three patients likewise had a laterally subluxed patella pre-operatively. The intrinsic ‘effective’ 3⬚ external rotation therefore was used as a starting point, which, for the majority of knees, was an end point as well. When the leg was positioned for the tunnel X-ray, the weight of the lower leg was used to apply a mild distractive force to the knee joint. We used this radiographic technique to supplement our clinical impression of varus᎐valgus stability when the knee was flexed. In 91 knees, the medial and lateral spaces were equal in size. The purpose of a lateral retinacular release is to allow the patella to track normally in the trochlear groove without tilt or subluxation. The incidence of lateral release procedures has varied widely in the literature w3,8x and this variation is based upon multiple factors. For example, the type of pre-operative deformity can influence the rate of lateral release procedures, being greatest in patients with a fixed valgus contracture and those with a pre-operative subluxation of the patella. The surgical incision has been implicated as well. Engh w2x described a 70% reduction in the requirement for lateral release procedures when he changed from a median parapatellar to a mid vastus capsular incision. Femoral component design has an obvious effect as well. Femoral components with a high conforming lateral trochlear ridge cause increased tension on the lateral retinaculum and increase the rate of lateral tilt and subluxation. Likewise, trochlear surfaces that are completely flat have resulted in a very high incidence of lateral subluxation and dislocation. Finally, patellar preparation has an effect on patellar tracking and the necessity for a lateral retinacular release. If insufficient patellar bone is removed, the composite bone and implant are thickened relative to the pre-operative state and this causes not only increased tension in the lateral retinaculum but increased contact stresses in the polyethylene as well. In the normal patella, the lateral facet is larger than the medial facet, so that the patellar ridge is medial to
R.S. Laskin r The Knee 6 (1999) 87᎐93
the mid-point of the bone. This should be reproduced in the prosthetic patella as well and is the rationale for medialization of the symmetrical patellar implant. In a patient with a patella that is much larger mediolaterally than it is antero-posteriorly, this may result in an uncovering of the patellar bone laterally which can come in contact with the femoral trochlear surface. There is no uniform opinion as to whether the most lateral portion of the lateral facet should be removed. Possibly, the patellar meniscus, which almost routinely covers the periphery of the patellar implant and the bone, may protect against impingement and anterior knee pain. Despite the lower lateral release rate noted in this study Ž3%., there was no compromise in the post-operative patellar tracking as measured by patellar tilt and patellar shift on the post-operative Merchant views. The mean lateral tilt was just over 1⬚ with over 84% of the knees having no tilt at all. Patellar tilt and or subluxation, when it does occur, is usually manifested within the first 6 months after the arthroplasty. For that reason, it is felt that this 1-year roentgenographic evaluation is sufficiently sensitive to evaluate this phenomenon. Likewise, although there have been no prior studies roentgenographically evaluating the flexion space post-operatively, it would seem that if there were instability due to the surgical procedure, itself too would be manifest by the period of time after surgery. In measuring patellar tilt, the orientation of the transverse axis of the patella, as described by Grelsamer et al. w10x was compared to a line joining the anterior medial and antero lateral edges of the femoral trochlear groove. A second measurement was made for each patient to determine the patellar implant tilt. In this case, we measured the angulation between the undersurface marker wire of the patella and the anterior condylar line. Both of these measurements are important, the former in ascertaining the overall tracking of the patella with its implant, and the latter in ascertaining the proper placement of the implant in the bone. We did not measure the patella angle as reference to the posterior femoral condyle nor did we elect to measure the congruence angle in these patients. Not only has Inoue shown that congruence angle measurements are not sensitive to measure patella subluxation, a reproducible method of measuring these angles in the presence of a knee implant could not be made. Anterior knee pain after surgery can have multiple etiologies. It is important when evaluating this pain to exclude those situations in which the lateral numbness, often seen for many months after an anterior midline incision, not to be confused with pain. The
93
commonest causes that we had found in the past for anterior knee pain were maltracking of the patella, overstuffing of the patello-femoral joint space and avascular necrosis of the patella. True anterior knee pain is most prominent with ascending or descending stairs or ramps, when arising from a chair, and when kneeling. Objectively it can be reproduced by direct pressure on the patella against the trochlear groove. The incidence of anterior knee pain in this study was only 2%. 5. Summary Using a neutrally positioned femoral component with asymmetrical posterior condyles, we were able to balance the flexion space without having to externally rotate the implant. Three percent of the patients required a lateral release. Patellar tracking and flexion space balancing were correct in the overwhelming percentage of patients, and the incidence of anterior knee pain was 2%. We feel that this surgical technique and implant has obviated the problems associated with external rotation of the femoral component which were seen when a symmetrical femoral component was used. The lateral release rate and the residual anterior knee pain we observed are lower than those reported in other published literature. References w1x Anouchi YS, Whiteside LA, Kaiser AD Milliano MT. The effects of axial rotation alignment of the femoral component on knee stability and patellar tracking in total knee arthroplasty as demonstrated on autopsy specimens. Clin Orthop 1995;287:170᎐177. w2x Engh GA, Parks NL, Ammeen DJ. Influence of surgical approach on lateral retinacular releases in total knee arthroplasty. Clin Orthop 1998;331:56᎐63. w3x Grace JN, Rand JA. Patellar instability after total knee arthroplasty. Clin Orthop 1988;237:184᎐189. w4x Insall JN, Windsor RE, Scott WN, Kelly MA, Aglieti P. Surgery of the knee. New York: Churchill Livingstone, 1993. w5x Laskin RS. Flexion space configuration in total knee replacement ŽAbstract.. J Arthroplasty 1995;10:657᎐661. w6x Moreland JR. Mechanisms of failure in total knee arthroplasty. Clin Orthop 1988;226:49᎐64. w7x Moreland JR, Bassett LW, Hanker GJ. Radiographic analysis of the axial alignment of the lower extremity. J Bone Joint Surg 1987;69A:745᎐749. w8x Rhoads DD, Noble PC, Reuber JD, Mahoney OM, Tullos HS. The effect of femoral component position on the kinematics of total knee arthroplasty. Clin Orthop 1998; 286:122᎐129. w9x Ries MD, Salehi A, Laskin RS, Bourne RB, Rand JA, Gustilo RB. Can rotational congruity be achieved in both flexion and extension when the femoral component is externally rotated in total knee arthroplasty ŽAbstract.. The Knee 1998;5:37᎐42. w10x Stiehl JB, Cherveny PM. Femoral rotational alignment using the tibial shaft axis in total knee arthroplasty. Clin Orthop 1996;331:47᎐55.