Patellar thickness and lateral retinacular release affects patellofemoral kinematics in total knee arthroplasty

Transcription

1 DOI /s z KNEE Patellar thickness and lateral retinacular release affects patellofemoral kinematics in total knee arthroplasty Azhar M. Merican Kanishka M. Ghosh Ferdinando Rodriguez Y. Baena David J. Deehan Andrew A. Amis Received: 8 July 2012 / Accepted: 19 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012 Abstract Purpose To study the effect of increasing patellar thickness (overstuffing) on patellofemoral kinematics in total knee arthroplasty and whether subsequent lateral retinacular release would restore the change in kinematics. Methods The quadriceps of eight fresh-frozen knees were loaded on a custom-made jig. Kinematic data were recorded using an optical tracking device for the native knee, following total knee arthroplasty (TKA), then with patellar thicknesses from -2 to?4 mm, during knee extension motion. Staged lateral retinacular releases were performed to examine the restoration of normal patellar kinematics. Results Compared to the native knee, TKA led to significant changes in patellofemoral kinematics, with significant increases in lateral shift, tilt and rotation. When patellar composite thickness was increased, the patella tilted further laterally. Lateral release partly corrected this lateral tilt but caused abnormal tibial external rotation. With complete release of the lateral retinaculum and capsule, the patella with an increased A. M. Merican Department of Orthopaedic Surgery, University of Malaya Medical Centre, Kuala Lumpur, Malaysia A. M. Merican A. A. Amis Musculoskeletal Surgery Group, Imperial College London, Charing Cross Hospital, London W8 6RF, UK a.amis@imperial.ac.uk K. M. Ghosh (&) F. R. Y. Baena A. A. Amis Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK miltonghosh77@hotmail.com K. M. Ghosh D. J. Deehan Department of Orthopaedic Surgery, Newcastle University Hospital, Newcastle-Upon-Tyne, UK thickness of 4 mm remained more laterally tilted compared to the TKA with normal patellar thickness between 45 and 55 knee flexion and from 75 onwards. This was on average by 2.4 ± 2.9 (p \ 0.05) and 2. 9 ± 3.0 (p \ 0.01), respectively. Before the release, for those flexion ranges, the patella was tilted laterally by 4.7 ± 3.2 and 5.4 ± 2.7 more than in the TKA with matched patellar thickness. Conclusion Patellar thickness affects patellofemoral kinematics after TKA. Although lateral tilt was partly corrected by lateral retinacular release, this affected the tibiofemoral kinematics. Level of evidence IV. Keywords TKA Patellar tracking Knee kinematics Lateral release Introduction Patellofemoral complications can cause poor functioning after total knee arthroplasty (TKA) [5, 9, 23]. There is good experimental and clinical evidence that femoral component rotational misalignment can affect patellar kinematics adversely [3, 5, 35], leading to an increased rate of lateral releases [1, 38]. Similarly, medialisation of the patellar component on the patella has been recommended to improve patellar tracking [43] and has been shown to reduce the need for lateral releases [29]. The effect of the thickness of the reconstructed patella on patellar tracking has been investigated less than the effects of component rotations and positions. Although it has been advocated that a thicker patellar bone prosthesis construct than the native patella ( overstuffing ) should be avoided [23, 27] and is logical, there are no studies that document a relationship between patellar thickness and lateral release rates.

2 Patellar tracking during surgery has been grossly visualised to be unchanged when the patella was up to 8 mm more than the thickness of the intact patella [4]. A radiological study did not find a significant effect of the thickness of patellar resection on patellar tracking after knee replacement [22]. In contrast, another study [26] found a correlation between the change in patellar thickness and post-operative tilt. This finding is consistent with in vitro work that showed a thicker patella increased lateral tilt [17]. In the natural knee, lateral retinacular release did not change patellofemoral contact area or pressure [15, 28]. Lateral release following a TKA, however, can result in medial tilting of the patella in extension and early flexion [18]. The aim of this study was to investigate the effects of a range of thicknesses of the patellar construct on the kinematics of the patellofemoral joint after TKA. Secondly, because increasing patellar thickness tightens the retinacular structures, the influence of lateral retinacular release on restoration of normal kinematics was examined. We made two hypotheses; first, that increased patellar thickness (overstuffing) would cause patellar lateral tilting, and second, that lateral retinacular release would restore the tilt towards normal. Materials and methods Eight fresh-frozen cadaveric knees were used (mean age 63? 16 years SD). They were obtained from a tissue bank which undertook serological screening and consenting. The study was approved by the Riverside Research Ethics Committee. The knees were stored at -20 C prior to experimentation. Kinematics A Polaris optical system (Northern Digital Incorporated, Waterloo, Canada) was used with active optical trackers (Fig. 1). The knee was moved into two cycles of flexion extension, against the extending moment of the tensioned quadriceps. The femur coordinate system was centred on Fig. 1 The experimental setup. A right knee is placed in the jig, and the various components of the quadriceps are loaded. Optical trackers are secured to the femur, patella and tibia

3 the anatomical axis and rotationally aligned with the most posterior points of the femoral condyles. The centre of the patella was defined as 10 mm deep to the anterior surface of the patella and overlying the midpoint of the median ridge. The patellar coordinate system was constructed from this origin and aligned to the medial, lateral and distal points [20]. The tibial coordinate system was based on the anatomical axis and the most medial and lateral points of the tibial plateau. The kinematic data were processed with Visual3D software (C-Motion Inc., Maryland, USA) using a standard joint coordinate system for the knee [14]. From this, the kinematics of the patella in relation to the femur were determined. Patellar tilt was defined as a rotation about the longitudinal axis of the patella, positive values indicating that the lateral patella approached the femur. Patellar lateral rotation was a rotation of the patella in its own coronal plane into abduction. Lateral translation was a movement perpendicular to the anatomical axis of the femur and parallel to the plane which passed through the most posterior points of the femoral condyles. All of these variables were set to zero at 0 tibiofemoral flexion (Fig. 2). Surgical technique The knees were prepared and mounted on a kinematics rig in a manner previously published [12]. The quadriceps and iliotibial band (ITB) were loaded proportionately in their physiological directions with tensions of 175 and 30 N, respectively [11, 24, 32]. The quadriceps was separated into six components: rectus femoris (RF), vastus intermedius (VI), vastus lateralis longus (VLL), vastus lateralis obliquus (VLO), vastus medialis longus (VML) and vastus medialis obliquus (VMO). The quadriceps tension distribution was as follows: RF?VI 35 %, VLL 33 %, VLO 9 %, VML 14 % and VMO 9 % muscle component. In order to preserve the soft tissue envelope and reduce confounding factors such as variable suture tensioning affecting retinacular behaviour after repeated opening and closing of a standard medial parapatellar approach, a longitudinal transpatellar approach was used; this was known not to affect the retinacula [33]. A posterior cruciate ligament-retaining TKA was used (Genesis II, Smith & Nephew, Memphis TN). The implant had a thinner posteromedial femoral condyle compared to the lateral side, and so a neutral cut based on the posterior condyles balanced the flexion gap without additional external rotation of the femoral component. Sizing of the femoral component used anterior referencing in order to preserve the trochlear thickness. The tibia was cut perpendicular to the anatomical axis and with a 3 posterior slope using an extramedullary jig. A polyethylene patellar socket was cemented into the anterior patella so that an optical tracker could be mounted on it. The tracker could be removed and returned to its original position. The socket was accurately cemented so that it was perpendicular to the anterior surface of the patella and in line with the centre of the patellar median ridge. This was because the cemented polyethylene socket also aligned the patellar component later. The Fig. 2 Definition of patellar motion (Adapted with permission from Bull et al. [7])

4 thickness of the native patella was measured from the centre of the median ridge to the anterior surface using a digital caliper with 0.01 mm resolution. The patella was then cut using a jig so that the same thickness could be restored with the patellar prosthesis and a 2-mm polyethylene shim. The patellar component was secured by a central screw which passed through the cemented patellar socket and the patella (Fig. 3). The patellar split was closed using two cannulated cancellous screws placed proximally and distally across the patella through predrilled holes [33]. The split in the quadriceps tendon was approximated without tension using a suture. Kinematic measurements were repeated as described earlier. The knee was opened again and the 2-mm shim underneath the patella was removed, so that the bone prosthesis patellar composite thickness was 2 mm less than the native patella (P2- group). The knee was closed again, and the kinematic measurements were repeated. Thereafter, the patellar thickness was increased to 2 mm (P2? group) and then 4 mm (P4? group) above the native patellar thickness by the addition of 2 mm shims. The lateral release was performed in 3 stages from outside-in [40, 42]. First, the lateral retinaculum was incised longitudinally, lateral to the patella, from 20 mm proximal to the proximal end of the patella to the distal pole of the patella. The vastus lateralis obliquus (VLO) tendon that attached to the widest (medial lateral) part of the lateral patella was not cut but instead the connection between the iliotibial band (ITB) and the distal edge of this tendon was disrupted. This proximal release of the lateral retinaculum cut through the thickest part, with the deeper transverse fibres that connected the ITB to the lateral patella (ITB-P fibres) [31] and the VLO, but left the joint capsule intact. The second stage completed the release of the lateral retinaculum distally, alongside the patellar tendon to the level of Gerdy s tubercle. In the third stage, the lateral capsule was incised, transecting the thickenings of the capsule: the lateral patellofemoral and patellomeniscal ligaments [31]. Statistical analysis Each component of the kinematic data for the various test conditions was analysed statistically at every 5 knee flexion using a two-way repeated measures analysis of variance across the range of tibiofemoral flexion angles between the knee replacement and the various thicknesses of the patellar replacement and the stages of lateral releases. A Bonferroni post hoc test was used to determine the testing conditions and knee flexion angles when a statistically significant difference had occurred. Differences were taken to be significant for p \ Fig. 3 Placement of the polyethylene patellar socket. a Using an anterior cruciate ligament targeting jig and a drill block resting on the anterior surface of the patella, a wire is passed tangential to the transverse and long axis of the patella centred over the proximal distal centre of the median ridge. b A cannulated drill was used to create a larger hole in the same alignment as the wire. c A polyethylene socket is cemented into the patella. d There are screw holes in the socket that can be used to secure a polyethylene disc with the attached patellar tracker. e After the kinematic testing of the intact knee, the tracker can be removed for surgical implantation of the prosthetic patella. The patella is resurfaced, and the prosthesis is secured with a countersunk screw that passes into the polyethylene socket. This technique allows for accurate positioning over the centre of the median ridge and good alignment of the patellar prosthesis. The patellar tracker is returned to its original position by fixation of the disc to the socket Results Effect of TKA on patellar lateral tilt In the native knee, the patella was tilted laterally by 6.3 ± 2.9 relative to the femoral posterior condylar axis in the extended knee. From there to 15 of flexion, there was a reduction in the lateral tilt to 4.3 ± 1.2, followed by an increase in tilt with further knee flexion to 12.3 ± 1.5 at 100 flexion. With a TKA with a thicknessmatched patella, there was a significant increase in lateral patellar tilt between 10 and 45 knee flexion; this was maximal at 20 flexion with 6.7 ± 5.1 (p \ 0.05) more lateral patellar tilt than in the native knee (Fig. 4).

5 Effect of alteration in patellar thickness on patellar lateral tilt Increasing patellar thickness after TKA ( overstuffing ) increased the lateral patellar tilt. A statistically significant difference was not observed with the patellar thickness reduced by 2 mm (P2-) or with the patellar thickness increased by 2 mm (P2?) (ns not significant). With the P4?, patellar tilt was significantly increased (p \ 0.001) from 30 onwards (Fig. 5; Table 1). Significant differences were not found in all the other kinematic degrees of Fig. 4 Patellar lateral tilt for the native knee and after TKA. Mean and SD, n = 8 Table 1 Difference in patellar tilt angle compared to knee replacement with matched patellar thickness at different knee flexion angles Knee flexion Difference in tilt from TKR P4? Release Mean SD Mean SD ns ns ns ns ns ns ** ns ** ns *** ns *** ns *** ns *** ns *** * *** * *** * *** ns *** ns *** ns *** * *** ** *** ** *** *** *** *** *** *** After increasing the patellar thickness by 4 mm (P4?) and then subsequently after complete release of the lateral retinaculum and capsule (ns = p [ 0.05; * p \ 0.05; ** p \ 0.01; *** p \ 0.001) freedom after altering the thickness of the patellar component from that of the original TKA. Effect of lateral retinacular releases on patellar lateral tilt Fig. 5 Patellar lateral tilt after TKA, reducing the patellar thickness by 2 mm (P2-), increasing by 2 mm (P2?) and increasing by 4 mm (P4?). Mean and SD, n = 8 The increased lateral patellar tilt that resulted from increasing the patellar thickness by 4 mm was reduced towards values for that of the TKA with normal patellar thickness with successive release of the lateral patellar restraints (Fig. 6). With complete release of the lateral retinaculum and capsule, the patella with an increased thickness remained more laterally tilted compared to the TKA with normal patellar thickness between 45 and 55 knee flexion and from 75 onwards. This was on average by 2.4 ± 2.9 (p \ 0.05) and 2.9 ± 3.0 (p \ 0.01), respectively. This is in contrast to before the release (P4?) in which for those flexion ranges, the patellar was tilted laterally by 4.7 ± 3.2 and 5.4 ± 2.7 more than in the TKA with matched patellar thickness (Table 1).

6 Fig. 6 Patellar lateral tilt after TKA, with increased patellar thickness 4 mm (P4?), then after successive lateral retinacular releases (LRR) and capsular release. Mean and SD, n = 8 Fig. 7 Tibial external rotation after TKA, with increased patellar thickness 4 mm (P4?), then after successive lateral retinacular releases (LRR) and capsular release. Mean and SD, n = 8 The significant change in lateral patellar tilt observed with the increased patellar thickness from 15 knee flexion onwards was reduced so that between 15 and 40 (ns) and between 60 and 70 (ns), a significant difference was not observed between the overstuffed TKA plus complete lateral retinacular release and the TKA with normal patellar thickness. Other kinematic effects on the patellofemoral joint Insertion of the TKA shifted the patella medially, by 3.9 ± 2.5 mm at 0 knee flexion, increasing to 6.0 ± 3.3 mm at 95 flexion. The patellar medial lateral position was not changed significantly by altering the thickness of the patellar component. Lateral retinacular release did not cause significant changes of patellar medial lateral translation. Changes in thickness of the patella led to the expected alterations in patellar position. With the knee in extension, the differences in the anterior position of the patella -2,?2 and?4 mm compared to the TKA group were on average -2.2 mm (p \ 0.01, 95% CI -0.6 to -3.8),?1.7 mm (p \ 0.05, 95% CI ) and?3.6 mm (p \ 0.001, 95% CI ). Effects on tibial internal external rotation This work caused changes in the internal external rotation of the tibiofemoral joint. Insertion of the TKA reversed the screw-home motion, from 4.9 ± 3.0 of external rotation to 6.4 ± 2.7 internal rotation in the last 20 of extension. Altering the thickness of the patellar component had no effect on tibial rotation, but the lateral releases allowed the tibia to be pulled into external rotation by the ITB tension, when the knee was flexed (Fig. 7). When the proximal part of the retinaculum was released, there was a 3.7 ± 1.4 increase in tibial external rotation at 60 flexion, which increased to 5.6 ± 3.8 and 7.4 ± 4.0 when the release also included the distal retinaculum and the capsule, respectively. Discussion This study shows that increasing the thickness of the prosthetic patella (overstuffing) during TKA caused lateral tilting. The patellar lateral tilting which followed from overstuffing the joint had been expected from prior understanding of the anatomy and mechanics of the tissues. The medial patellofemoral ligament (MPFL) is thin and has low strength and stiffness [36], while the transverse fibres which connect the patella to the ITB are considerably stronger and stiffer [34]. Thus, when the retinacula were tensed by overstuffing the joint, the lateral structures tensed by the iliotibial band tethered the lateral edge of the patella, while the medial edge lifted, causing lateral tilt. This difference in tensile elongation has been reported [13]. In addition, the patellar button was centred over the median ridge, thus giving a relatively medial pivot point, about which patellar tilt could occur. Lateral tilting was increased by moving the patellar button medially [2]. However, lateral tilting may still occur when the patellar prosthesis is placed centrally [17]. Even when the precut thickness of the patella was restored to equal that of the native knee, kinematics were not restored. The resurfaced patella was found to tilt 7 laterally compared to the native knee from 10 to 45 knee flexion. This behaviour may be attributed to a mismatch in implant and native anatomy. We found the resurfaced patella to be displaced anteriorly in the extended knee post TKA, because of the increased thickness of the anterior

7 flange of the prosthesis. We assume this mismatch is an alternative mechanism of overstuffing the patellofemoral joint, thereby starting the process of stretching the retinacula beyond their intact state and causing lateral tilt. A similar mechanism has been suggested by Hollinghurst et al. [16] after patellofemoral arthroplasty. Laboratory and clinical studies have found similar patterns of patellar tilting post TKA. The lateralised trochlear groove of the Genesis II prosthesis near extension was designed to facilitate patellar tracking, and lower lateral release rates have been reported with the Genesis II compared to the Genesis I [21]. Good intraoperative subjective assessment and post-operative radiological assessment of patellar tracking have been reported with this implant [25]. It should be noted, though, that tilting is assessed from skyline views post TKA in vivo, and the rotational alignment of the prosthetic trochlea on which those measurements are based may differ from the methods used in vitro. Further, those knees in which lateral tilt was noted intraoperatively may have had a lateral release, which would have biased the results. Other studies, both in vitro [8] and in vivo [41] found significant lateral tilting of the patella post TKA compared to the natural knee, with values similar to the present work. As had been hypothesised, staged lateral retinacular release did reduce patellar lateral tilt. This, however, was not the same as returning the kinematics to normal. Increasing the thickness of the patella by 4 mm led to an additional 6 of lateral tilt over and above an already tilted resurfaced patella (as above). Complete lateral retinacular release reduced this abnormal tilt by approximately 3, still leaving the overstuffed patella significantly tilted when compared with the natural path of motion. Although this study found that lateral retinacular releases reduced patellar lateral tilting post TKA, the situation remains controversial in the clinic, and published studies of TKA have reported release rates from 3 % [27] to50% [42]. Assessment of patellar tilt intraoperatively and the decision to release the lateral retinacula are hampered by factors including lack of muscle tension due to anaesthesia, restraint from a tourniquet [19, 30] and transection of medial peripatellar tissues by the approach [6, 10]. Our final findings appeared to show that the increase in lateral tilt was greater with knee flexion and that lateral release had a greater effect in decreasing tilt at these higher flexion angles. This is in keeping with the observation in TKA surgery where a tight retinaculum is appreciated as a patella which tilts laterally when the knee is flexed to 90. This reflects the normal behaviour of the retinacula: the MPFL remains isometric, whereas the lateral transverse fibres are stretched significantly with knee flexion [36, 39]. An important distinction of this study is that the iliotibial band (ITB) was also loaded, tensing the lateral retinaculum and influencing the patellofemoral contact and kinematics [24, 32]. There are, however, necessary limitations of this study. Although the quadriceps and ITB were loaded as realistically as possible, hamstrings loading was omitted, but to minimise the effect of co-contraction of the hamstrings, only knee extension motion was analysed. Previous works have underscored the importance of the oblique components of the quadriceps and multiplanar muscle loading on patellar mechanics [11, 37]. Although care was taken to tense the individual heads of the quadriceps in physiological directions, their relative contributions may vary from knee to knee and the overall tension was limited by tearing the muscle fibres in the elderly specimens. Conclusion This paper provides biomechanical evidence that overstuffing the patellofemoral joint during TKA caused lateral patellar tilting and that this maltracking could be only partly corrected by lateral retinacular release. However, the patella was tilted laterally even when its thickness post TKA matched the intact value, and that may relate to the position and geometry of the patellar button and to the thickness of the anterior flange of the femoral component. Previous work which addressed the influence of lateral release in TKA did not simulate a tightened retinaculum, nor load the ITB [32] and thus may have underestimated these effects. Patellar maltracking post TKA varies in severity, and therefore lateral release should be tailored for the desired effect. This paper shows that progressive reduction in lateral tilting is possible with a staged partial release. Acknowledgments Azhar M. Merican was supported by the University of Malaya Research Grant and by a project grant from Arthritis Research UK. K. Milton Ghosh was supported by Smith & Nephew reconstruction (UK), who also donated the implants. We thank W. Scott Selbie, PhD, Director of Research and Development, C-Motion, Inc., for his invaluable help and software support. Conflict of interest References The authors have no other conflicts of interest. 1. Akagi M, Matsusue Y, Mata T, Asada Y, Horiguchi M, Iida H, Nakamura T (1999) Effect of rotational alignment on patellar tracking in total knee arthroplasty. 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