Length Change Patterns of the Extensor Retinaculum and the Effect of Total Knee Replacement

Transcription

1 Length Change Patterns of the Extensor Retinaculum and the Effect of Total Knee Replacement Kanishka M. Ghosh, 1,2 Azhar M. Merican, 3,4 Farhad Iranpour-Boroujeni, 3 David J. Deehan, 2 Andrew A. Amis 1,3 1 Mechanical Engineering Department, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom, 2 Orthopaedic Surgery Department, Newcastle University Hospital, Newcastle upon Tyne NE7 7DN, United Kingdom, 3 Musculoskeletal Surgery Department, Imperial College London, Charing Cross Hospital, London W6 8RF, United Kingdom, 4 Orthopaedic Surgery Department, University of Malaya Medical Centre, Kuala Lumpur, Malaysia Received 13 August 2008; accepted 7 November 2008 Published online 8 January 2009 in Wiley InterScience ( DOI /jor ABSTRACT: Patellofemoral dysfunction following total knee replacement (TKR) is a significant clinical problem, but little information exists on the mechanics of the patellofemoral retinacula or the effects of TKR on these structures. We hypothesized that TKR would cause significant elongation of the retinacula. Retinacular length changes were measured by threading sutures along the retinacula, fixing the sutures to the patella and the iliotibial band (ITB), and attaching the femoral ends to displacement transducers. The intact knee was flexed-extended while the quadriceps and ITB were tensed and the retinacular length change patterns were recorded. The measurements were repeated post-tkr. The medial patellofemoral ligament (MPFL) was close to isometric, stretching 2 mm in terminal knee extension, whereas the lateral retinaculum slackened 8 mm from 1108 to 08 flexion. TKR did not cause significant elongation of either of the retinacula, the largest change being 3 mm elongation of the MPFL around 408, which stretched the MPFL by 1.4 mm above its maximum natural length. Thus, this work did not support the hypothesis that TKR causes significant elongation of the retinacula sufficient to affect knee function. ß 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: , 2009 Keywords: knee; patellofemoral; retinaculum; TKR; length change patterns Demand is increasing for improved knee function after surgery. Despite continuing evolution in prosthesis design and surgical methods, stiffness after total knee replacement (TKR) remains a disabling complication. The prevalence appears to be relatively low, but has not been clearly defined, primarily due to ambiguity in the literature concerning its definition. The prevalence of knee stiffness was recently reported as 1.3% 3.7%, 1,2 though higher levels (8% 12%) were reported in older literature. 3 Stiffness after TKR may be caused by limited preoperative motion, biological predisposition, intraoperative technical problems, poor patient motivation, and inadequate postoperative rehabilitation. 1,4-6 During knee motion, tensions in the ligaments, capsule, and extensor envelope may restrict flexion and affect implant wear. 6,7 Awareness of the role of the extensor retinaculum in patellofemoral kinematics is increasing. 8 While studies to date have focused on its role in patellar instability, 8 11 the influence of TKR is unknown. The positioning and thickness of TKR components may cause the surrounding soft tissues to be stretched. Abnormal soft tissue tensions post-arthroplasty may alter patellofemoral kinematics and stability; the literature shows that these mechanical factors can lead to clinical problems such as pain, stiffness, and excessive wear. Our aims were: to measure the length change patterns of the medial and lateral retinacula during knee extension; and to test the hypothesis that insertion of a TKR would cause significant elongations of the retinacula. Correspondence to: Andrew A. Amis (T: þ ; F: þ ; a.amis@imperial.ac.uk) ß 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. MATERIALS AND METHODS Specimen Preparation Eight right knees aged 69 years (range, years) and including 20 cm of femur and 15 cm of tibia were obtained from a tissue bank following ethical approval. They were sealed in polyethylene bags and frozen at 208C prior to use. They were thawed in a refrigerator at 38C for h, kept moist using normal saline, and then prepared as in previous work. 12 The skin, underlying fat, and muscles other than the distal quadriceps were removed. Care was taken to preserve the retinacula, quadriceps muscles fasciae, and the iliotibial band (ITB). The quadriceps were then 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 distal muscle fibers merged together and were left intact to ensure that their actions were as physiologic as possible. The VI was separated from the femur. Cloth strips were looped over and sutured to the proximal end of each muscle and the ITB to provide attachment for a muscle loading cable. The RF and VI were looped together to form a central muscle group. The head of the fibula was fixed to the tibia by two bone screws, to maintain its anatomical position. A brass intramedullary sleeve was secured into the femur, and an intramedullary rod into the distal tibia, using polymethylmethacrylate (PMMA) bone cement. Experiment Set-Up The knee specimen was mounted into the testing rig (Fig. 1) by sliding the intramedullary brass sleeve over a fixed rod and clamping tightly. This allowed axial rotation of the femur, ensuring that a line joining the most-posterior points of the condyles was horizontal. The muscle alignment system was adjustable, allowing each muscle head to be loaded in its physiologic direction relative to the femoral axis. 13,14 A 175 N load was applied to the quadriceps muscle groups by hanging fixed weights on a cable and pulley system. The ITB was loaded to 30 N, 08 lateral and 68 posterior to the femoral axis. 15,16 The 865

2 866 GHOSH ET AL. Figure 1. Knee mounted in kinematics rig with motion sensors attached. The hand-held probe is on the femoral medial epicondyle. The displacement transducers are mounted in the triangular blocks on the base. The vastus medialis obliquus (VMO) is tensed by the string passing over the pulley, central, right. Vastus medialis longus (VML) and rectus femoris (RF) are at top, right. quadriceps tension distribution was according to the mean physiologic cross-sectional areas of the muscles. 13 Data Collection Tibiofemoral flexion was measured using a Polaris Optical Tracking System (NDI Intl., Waterloo, Canada). One optical tracker was placed on the femur and another on the tibia. The fully extended position was defined as when the tibial intramedullary rod was parallel in the sagittal plane to the femoral anatomical axis. 17 Knee flexion was calculated using Visual 3D (C-Motion Inc, Rockville, MD). Length changes were measured in the principal structures of the medial and lateral parapatellar retinacula. On the medial side, the most important soft tissue structure for patellar stability is the medial patellofemoral ligament (MPFL). 10,18,19 The MPFL was found in all eight specimens running transversely between the proximal half of the medial border of the patella to the femur immediately proximal to the medial epicondyle in the second tissue layer, below the superficial fascia and above the capsule. 11,20 The lateral retinaculum is more complex, because of the overlying ITB passing alongside the patella forming a multilayered structure. 21,22 The deep aspect of the ITB acts as the origin for a distinct transverse lateral retinacular structure that attaches to the mid-lateral edge of the patella. 22,23 Often called the lateral patellofemoral band, it is not attached to the femur, except indirectly via the proximal and distal ITB attachments. 21,22,24 It is this deep transverse band that is transected in a conventional open lateral release procedure, so this was selected for length change measurements. Length changes were measured using a monofilament suture attached to the sliding core of a linear variable displacement transducer (LVDT; Solartron Metrology, Bognor Regis, UK; 1 mm resolution) (Fig. 2), and device-specific software ( Orbit Excel, Solartron Metrology). The suture was kept under constant low tension (about 0.2 N) by attaching a rubber band to the LVDT core. A single monofilament suture (Ethilon 2/0, Ethicon, Somerville, NJ) was used to measure length changes in the MPFL. A suture was passed from the LVDT through a screw eye secured at the origin of the MPFL at the mid-point of its width in the deepest part of the sulcus between the medial epicondyle and the adductor tubercle. The suture was then threaded along the fibers of the ligament and secured at its attachment to the superomedial corner of the patella. This method has been used Figure 2. Diagram of the lateral aspect of the knee, showing the paths of sutures from the linear variable displacement transducer (LVDT) to the lateral retinaculum. One suture end is fixed to the eyelet on the iliotibial tract adjacent to the lateral epicondyle; the other suture passes through the eyelet and under the tract to attach to the lateral edge of the patella. Differences between the movements of the two sutures give the length change of the transverse fibers of the lateral retinaculum.

3 EXTENSOR RETINACULUM LENGTH CHANGES 867 to measure length changes in the fibers of the anterior cruciate ligament. 25 Because the deep transverse band in the lateral retinaculum originates and inserts into two mobile structures (ITB and lateral patella), it was necessary to measure motion of the two structures separately to calculate the length change of the fibers in between (Fig. 2). With the quadriceps loaded and the knee extended, the palpably thick transverse band lay in a straight line from the superolateral corner of the patella, passing posteriorly to the deep aspect of the ITB. Movements were measured along one suture passing from an LVDT through an eyelet sutured into the ITB to the patella along the transverse band of fibers. A second LVDT measured movements with a suture passing to the ITB eyelet only. Thus, length change in the deep transverse band was calculated by subtracting the two values. Before testing, 10 preconditioning cycles of passive flexionextension were performed against the extending action of the quadriceps tension. This was done by pushing posteriorly against the tibial intramedullary rod, using a nylon rod handheld transversely across the end of the rod to avoid imposing secondary moments. Measurements were recorded while the knee was allowed to extend slowly from 1108 flexion to extension. Testing was first conducted with the knee intact and then with a standard TKR installed in neutral rotation. Total Knee Replacement To preserve the retinaculum and reduce confounding factors such as variable suture tensioning from affecting ligament length changes after closure of a standard medial parapatellar approach, a transpatellar approach extending 5 cm proximally into the quadriceps tendon and 3 cm distally into the patellar tendon in the line of fibers was used to access the joint. 26 This approach does not cause significant length changes in the medial and lateral retinacula. A cruciate retaining TKR (Genesis II, Smith & Nephew, Memphis, TN) was used. The procedure was performed by the same consultant surgeon on all eight knees. The patella was resurfaced using a dome-shaped onlay button centered on the median ridge. The thickness of the patella was measured preand post-tkr using vernier calipers to ensure that the original thickness was restored to within 0.5 mm. A standard surgical protocol and instruments were used for component implantation as recommended by the implant manufacturer. Anterior referencing femoral instrumentation was used to ensure restoration of the thickness of the patellofemoral joint. If the indicated size fell between two sizes, the smaller size was chosen to avoid overstuffing of the flexion space. As the distal half of the tibia was absent, the tibial cut was made using an extramedullary guide aligned with the intramedullary rod. Flexion and extension gaps were assessed subjectively in extension and 908 flexion. The Genesis II system uses a modified femoral component with a thicker posterior-lateral than posteriormedial condyle. This enabled the implant to have 38 external rotation from the posterior condylar axis and maintain an equal flexion gap without the need to remove excess bone from the posteromedial condyle. The component was positioned at 38 external rotation from the posterior condylar axis by ensuring the posterior condyles of the implant were in the same position as the native knee; this was confirmed to correlate with the epicondylar axis throughout the work and was defined as neutral rotation. The transpatellar arthrotomy was closed using two cannulated cancellous screws placed through pilot holes drilled prior to the arthrotomy across the patella proximally and distally, without disrupting suture attachment points. The cuts in the quadriceps and patellar tendons were approximated without tension using Vicryl 2/0 sutures (Ethicon). Statistical Analysis A two-way repeated-measures ANOVA with Bonferroni posttests was used to determine differences in length of the MPFL and deep transverse band of the lateral retinaculum between the intact knee and TKR as the knee extended from 1108 to 08. Significance was set at p < RESULTS Length Changes in the MPFL of the Intact Knee The mean length change pattern for the MPFL in the intact knee was close to isometric (Fig. 3), with a mean tightening of mm ( 4.3 to þ4.7 mm) (mean SD; range) from 708 flexion to extension; this was not significant ( p ¼ 0.09). Length Changes in the Deep Transverse Band of the Lateral Retinaculum in the Intact Knee The lateral retinaculum slackened (Fig. 3) by mm ( mm) as the knee extended from 1108 to 08 ( p < ). Length Changes in the MPFL after TKR The mean pattern of length change in the MPFL after TKR remained close to isometric (Fig. 4), with a Figure 3. Length changes of the medial patellofemoral ligament (MPFL) and transverse fibers of the lateral retinaculum (LR) versus knee flexion with the intact knee. (Mean plus SD, n ¼ 8.)

4 868 GHOSH ET AL. Figure 4. Length changes of the medial patellofemoral ligament (MPFL) versus knee flexion after total knee replacement (TKR), compared with the intact knee. (Mean plus SD, n ¼ 8.) tightening of mm ( 1.1 to þ5.8 mm) as the knee extended from 90 8 to While the overall pattern of MPFL length was significantly different ( p < ) between the native knee and post-tkr, post-testing did not show significant differences at any specific flexion angle ( p > 0.05). The largest difference between the native knee and post-tkr was 3.1 mm at 40 8 flexion (95% CI of difference: 0.3 to 6.5 mm). The greatest stretching above the native length was 1.4 mm at 30 8 flexion. Length Changes in the Deep Transverse Band of the Lateral Retinaculum after TKR The lateral retinaculum was close to isometric when the knee extended from 1108 to 708 (Fig. 5), after which it slackened by mm (14.5 mm slackening to 0.5 mm tightening) as the knee reached full extension. An overall mean slackening of 0.4 mm occurred post- TKR ( p ¼ 0.01), ranging from 2.5 mm slack at 08 ( p < 0.001, 95% CI: 4.1 to 0.9 mm) to 0.4 mm tighter at 708 ( p > 0.05, from 108 to 1108). DISCUSSION This study yielded data on the length changes of both the medial and lateral patellar retinacula during knee extension and on the effects of TKR on those length change patterns. In the intact knee, there were contrasting length change patterns, between the medial and lateral retinacula. The MPFL was close to isometric across the range of motion, lengthening by a mean of 2 mm in the final 308 of knee extension. In contrast, the lateral retinaculum was longest in the flexed knee and slackened by a mean of 8 mm in the final 708 of knee extension. The implantation of a knee replacement did not cause significant stretching of the retinacula. The MPFL was 3 mm tighter than normal across the arc from 608 to 208 extension while the lateral retinaculum slackened 2 mm in the terminal 208 of knee extension. Neither change took the lengths of the retinacula above 2 mm extension beyond their normal maximum lengths. Thus, our findings refute the hypothesis that TKR Figure 5. Length changes of the lateral retinaculum (LR) versus knee flexion after total knee replacement (TKR), compared with the intact knee. (Mean plus SD, n ¼ 8.) causes retinacular elongation that might affect knee function. Although our results are related to this specific implant type and surgical technique, the authors have not found prior measurements of the length change behavior of the lateral retinaculum or of the effect of TKR on the lengths of either the medial or lateral retinacula. Several studies reported the length change pattern of the MPFL; the general consensus has been that it is desirable to create an isometric reconstruction. 8,27,28 However, a study of the effect of MPFL deficiency on patellar lateral stability 29 showed that the MPFL had its largest effect in the last 208 of extension. That led to the suggestion that it might be preferable to ensure that the MPFL graft was longest in extension, slackening in flexion. 30 That would avoid over-pressurizing the medial trochlea and fits with our data. The finding that the transverse band of the lateral retinaculum slackened by 8 mm when the knee was extended is in accordance with the motion of the ITB alongside the knee, moving anteriorly with knee extension and pulling posteriorly in flexion. Other work found that lateral retinacular tension rose to a maximum at 608 flexion in vitro, 9 or found a continuous rise in tension to 1208 flexion during surgery in vivo 31 ; these tension patterns match our length changes. After TKR, the length changes in the retinacula will be affected by the femoral condylar geometry and the tracking of the patella, so our study implies that TKR

5 EXTENSOR RETINACULUM LENGTH CHANGES 869 design and alignment did not significantly alter patellar tracking. Two prior studies 32,33 found that patellar tracking was not altered from the intact knee, examining the Genesis II (Smith & Nephew, Memphis, TN), NexGen (Zimmer, Warsaw, IN), and PFC Sigma (DePuy J&J, New Brunswick, NJ) prostheses. In four of the eight knees that we tested, the MPFL lengthened as the TKR knee extended to 408, then slackened towards full extension. Although this was not a significant effect for the whole group of knees, we presume that it reflects patellar tracking over the most-prominent anterodistal part of the femoral component. Again, our results relate to one implant design, and the femoral component was positioned carefully to match the original femoral geometry. Other prostheses, with different sagittal plane geometries, would likely cause different length change patterns in the retinacula, also as would different choices of component position and size. The main limitations of our study were that the experiments were performed on normal cadaveric knees without overt signs of degenerative disease. Thus, we could not simulate pathological changes in the soft tissues that may be relevant, such as tight bands in the lateral retinaculum. The presence of such pathology might magnify the changes produced in this experiment. The force applied to the quadriceps was limited by tearing of the muscles, and so normal physiologic magnitudes were not attained. In addition, the muscle tension was constant at all flexion angles, whereas, in life, the tension increases as we squat down. In addition, only the extensor muscles and ITB were loaded, so the effects of antagonistic muscle co-contraction on the retinacula remain unknown. This may have affected the length change patterns presented. In summary, a method to measure the length changes in the retinacula was developed. 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