Role of Peripatellar Retinaculum in Transmission of Forces Within the Extensor Mechanism

Transcription

1 2042 COPYRIGHT 2006 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED Role of Peripatellar Retinaculum in Transmission of Forces Within the Extensor Mechanism BY CHRISTOPHER M. POWERS, PHD, PT, YU-JEN CHEN, MS, PT, SHAWN FARROKHI, DPT, AND THAY Q. LEE, PHD Investigation performed at the Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System, Long Beach, California Background: The role of the peripatellar retinaculum as a frontal plane stabilizer of the patellofemoral joint has been well established. However, as a result of its unique orientation, the retinaculum also may influence the distribution of forces within the extensor mechanism. The objective of this study was to determine the extent to which the peripatellar retinaculum affects the magnitude of forces experienced by the patellar tendon. Methods: Ten cadaver knees were used in this investigation. Each was mounted on a custom test apparatus that was fixed to an Instron frame. The extensor mechanism was loaded by applying forces through the individual heads of the quadriceps femoris. Patellar tendon tension was measured at 0, 20, 40, and 60 of knee flexion with use of a buckle transducer under two conditions: (1) with the peripatellar retinaculum intact, and (2) with the peripatellar retinaculum removed. Patellar tendon tension was compared between the two conditions across the knee flexion angles. Results: At each knee flexion angle, the patellar tendon tension was greater with the retinaculum removed than it was with the retinaculum intact. However, the difference was significant only at 0 and 60, at which positions the force transmitted to the patellar tendon was increased by 16.6% and 9.6%, respectively. Conclusions: The observed increases in patellar tendon tension after removal of the peripatellar retinaculum is an indication of the load-sharing function of that structure as a part of the extensor mechanism. Clinical Relevance: Our results suggest that compromise of the peripatellar retinaculum may alter patellar tendon and/or patellofemoral joint forces. The mechanical function of the patella has been well described. As an integral part of the extensor mechanism, the patella acts as a spacer to provide a mechanical advantage for the quadriceps by increasing the distance between the muscle s line of action and the axis of rotation of the knee joint 1. Early biomechanical representations of the patellofemoral joint were created with the assumption that this articulation was a frictionless pulley system in which the patella acted as a linkage to transmit the quadriceps force to the patellar tendon without altering its magnitude 2-6. However, more recent biomechanical studies of the patellofemoral joint have demonstrated that the patella also acts as a complex lever system creating a force differential between the quadriceps tendon and the patellar tendon 7-10 that is, the patella has been thought to act as a balance beam, thereby altering the lengths of the quadriceps and patellar tendon moment arms as a function of the knee joint angle. The force differential between the quadriceps tendon and the patellar tendon is thought to occur as a result of the varying geometry and shape of the distal part of the femur and the patella as well as the changing point of contact between the patella and femur as the knee flexes and extends 1,8,9. Several in vitro studies have quantified the relationship between the force in the quadriceps tendon (F Q ) and the tension force in the patellar tendon (F PT ). The investigations have shown that the F PT /F Q ratio ranges from 1.0 to 1.2 with the knee fully extended to 0.6 to 0.8 with the knee flexed to Despite the variability in the reported F PT /F Q ratios among studies, all authors have reported a similar trend of a decreasing ratio (i.e., less patellar tendon tension relative to quadriceps force) with knee flexion. A limitation of previous studies is that it has been assumed that the force transfer from the quadriceps tendon to the patellar tendon is not influenced by the potential loadsharing behavior of soft tissues that cross the knee. One structure that has the potential for load-sharing between the quadriceps tendon and the patellar tendon is the peripatellar retinaculum. The normal peripatellar retinaculum consists of layered, fibrous connective tissue that traverses the medial and lateral margins of the patella with attachments to the femur,

2 2043 tibia, patella, and patellar tendon 13. Additionally, the superficial fibers of the retinaculum originate from the vastus lateralis and vastus medialis fascia, linking the quadriceps to the patella (Fig. 1) 13,14. This linkage is responsible for the dynamic influence of the quadriceps on the patellofemoral joint during active knee motion. The specific role of the peripatellar retinaculum as a frontal plane stabilizer of the patellofemoral joint has been well established However, as a result of its unique orientation, the peripatellar retinaculum also may play a complementary load-sharing role with respect to the patellar tendon. Similar to the patellar tendon, the peripatellar retinaculum provides distal inferior support for the patella through the medial and lateral patellomeniscal ligament tendons that connect the patella to the tibia 14. However, because of the transverse and oblique orientation of the retinaculum fibers, it is conceivable that this structure plays a role in providing medial-lateral stability for the patella and functionally unloading the patellar tendon by resisting tensile forces created by the extensor mechanism. The purpose of this study was to determine the extent to which the peripatellar retinaculum affects the magnitude of forces experienced by the patellar tendon. It was hypothesized that the peripatellar retinaculum acts as a load-sharing structure to decrease the forces experienced by the patellar tendon. Materials and Methods Experimental Setup en fresh-frozen, unmatched human cadaver knees were T used in this study. Each was macroscopically intact and radiographically normal. The donors of the specimens had ranged from sixty to eighty years of age at the time of death. After thawing, the knees were dissected with care taken to keep the retinaculum and the quadriceps tendon intact. Skin and subcutaneous fat were removed, as were the muscles from the tibia and the posterior part of the femur. The head of the fibula was secured to the tibia with a screw, while the distal two-thirds of this bone was removed. Approximately 20 cm of tibial and femoral length was left for mounting purposes. The individual components of the extensor mechanism, vastus medialis, vastus lateralis, vastus intermedius, and rectus femoris were separated from each other, with the fascial Fig. 1 Cadaver specimen demonstrating the complex anatomy of the human extensor mechanism. The superficial fibers of the peripatellar retinaculum originate from the quadriceps and have broad attachments to the patella, tibia, and patellar tendon. A: Sagittal view of the relationship between the vastus medialis and the medial retinaculum. B: Frontal view of the extensor mechanism. VM = vastus medialis, P = patella, RF = rectus femoris, ITB = Iliotibial band, and VL = vastus lateralis.

3 2044 planes between the muscles used as a guide. Following dissection, the muscles were trimmed to accommodate the width of the loading clamps. The muscle clamps measured cm and were made of stainless steel. Care was taken to select the portion of muscle that was representative of the resultant force direction of all muscle fibers. The muscles were clamped as close to their respective insertions as possible so that tendinous fibers could be incorporated within the clamp. The vastus intermedius and rectus femoris were clamped together since the direction of the resultant force vector of these muscles with respect to the patella is similar 19,20. Both the tibia and the femur were secured within 2- in (5-cm)-diameter polyvinyl chloride tubing with use of diaphyseal bolts and locking pins. Each bone was positioned within its tube such that the long axis of the cylinder represented the long axis of the bone. After the tibia and femur had been appropriately positioned, the plastic tubes were filled with dental plaster and the diaphyseal bolts were removed. The cylinders were then mounted on a custom knee test apparatus that provided six degrees of freedom at the femur and five degrees of freedom at the tibia 19,21. This apparatus was fixed to a materials testing machine frame (model 1122; Instron, Canton, Massachusetts), which was used to flex and extend the knee (Fig. 2). Anatomically positioned pulleys guided cables from the muscle clamps to the applied load. To quantify the force in the patellar tendon during testing, a buckle transducer (NK Biotechnical, Minneapolis, Minnesota) was placed near the tibial attachment (Fig. 2). The buckle transducer was calibrated after data collection by applying known collinear tensile forces through the patellar tendon. Recordings from the transducer were linear (R 2 = 0.99) throughout the range of calibration loads used (10 to 600 N). Muscle Loading As described in a previous publication 19, the extensor mechanism was loaded by applying forces through the individual heads of the quadriceps along their principal muscle fiber orientation. Loading of the individual muscles was important as the vastus medialis and the vastus lateralis have the potential to impart additional forces to the patella through their fascial expansion and interdigitation with the peripatellar retinaculum 14. The distribution of extensor force across the various muscles was based on cross-sectional area data reported by Fig. 2 Frontal view (A) and sagittal view (B) of the experimental setup used to quantify patellar tendon tension. The patellar tendon transducer was placed near the tibial attachment.

4 2045 Wickiewicz et al. (vastus medialis = 67 N, vastus lateralis = 98 N, and rectus femoris/vastus intermedius = 111 N) 22. Thus, the total extensor muscle force used in this study was 276 N and represented a submaximal load on the patellofemoral joint. Each pulley was adjusted so that the force application of the respective muscles represented the primary fiber direction and orientation (Fig. 2). Forces were applied to the respective muscle clamps through the cable system and were controlled with use of LabVIEW pneumatic cylinders (National Instruments, Austin, Texas). Experimental Procedures Each specimen was first positioned on the knee test apparatus in full extension. Care was taken to ensure that the proximal cylinder (femur) and the distal cylinder (tibia) were aligned so that the physiologic quadriceps angle (Q angle) was maintained. The Q angle was determined through the use of a standard goniometer prior to dissection. All degrees of freedom on the knee test apparatus were locked and the patellar tendon buckle transducer was zeroed prior to load application. Each pulley was then adjusted so that the force application to the respective muscles represented the primary fiber direction and orientation. Following the initial setup, muscle loading commenced and the buckle transducer force was recorded. After testing at 0 of knee flexion, all locking bolts controlling the degrees of freedom were loosened, and the knee was flexed to 20 by lowering the Instron crosshead. Following this procedure, the locking bolts were tightened, and the pulley systems were adjusted to accommodate the new knee flexion angle. The entire process, as described above, was then repeated. For each knee flexion angle of 0, 20, 40, and 60, the measurements of the patellar tendon force were repeated three times. Knee flexion angles were determined with use of a digital inclinometer. Once the tension of the patellar tendon was recorded with the retinaculum intact, the lateral and medial retinacula were removed. The lateral retinaculum was sectioned by performing a triangular incision beginning at the lateral patellar margin to the level of the tibial tubercle and extending proximally and laterally to the level of the lateral collateral ligament. Similarly, the medial retinaculum was removed with a triangular incision beginning at the medial patellar margin to the level of the tibial tubercle and extending proximally and medially to the medial collateral ligament. Following the removal of the retinaculum, the procedures, as described above, were repeated. Throughout all testing procedures, the specimens were sprayed with saline solution to keep the tissues moist. Statistical Methods A two-way repeated-measures analysis of variance was used to compare patellar tendon tension between the retinaculumintact and retinaculum-removed conditions across the knee flexion angles. Statistical analysis was performed with use of SPSS statistical software (SPSS, Chicago, Illinois) with a significance level of p < Results or all knee flexion angles tested, the mean patellar tendon F tension with the retinaculum removed was greater than the mean patellar tendon tension with the retinaculum intact (Fig. 3). Analysis of variance revealed a significant main effect Fig. 3 Comparison of the average patellar tendon tension between the retinaculum-intact and retinaculum-removed conditions as a function of knee flexion angle. The error bars indicate one standard deviation, and the asterisks indicate a significant difference.

5 2046 Fig. 4 Schematic drawing of the peripatellar retinaculum within the extensor mechanism. A: Lateral superficial layer. B: Lateral deep layer. C: Medial superficial layer. D: Medial deep layer. ITB = Iliotibial band, QT = quadriceps tendon, PT = patellar tendon, VM = vastus medialis, VL = vastus lateralis, SLR = superficial lateral retinaculum, LPFL= lateral patellofemoral ligament, TL = transverse ligament, LPT = lateral patellotibial band, MSR = medial superficial retinaculum, MPFL = medial patellofemoral ligament, PML = patellomeniscal ligament, and MPT = medial patellotibial ligament. for the condition (p = 0.008) and a significant condition knee flexion angle interaction (p = 0.031). Post hoc analysis consisting of paired t tests revealed that the mean patellar tendon tension with the retinaculum removed was significantly greater than that with the retinaculum intact both at 0 (206.0 ± 27.4 N compared with ± 34.9 N; p = 0.006) and at 60 (124.3 ± 18.0 N compared with ± 27.3 N; p = 0.018) (Fig. 3). There was no significant difference between the retinaculum-intact and retinaculum-removed conditions at 20 or 40 of knee flexion. Discussion he results of this study support the hypothesis that the T peripatellar retinaculum plays a role in the transmission of forces within the extensor mechanism. In particular, it was demonstrated that the patellar tendon experienced less tension with the retinaculum intact than it did with the retinaculum removed, under similar loading conditions (Fig. 3). When averaged across all knee flexion angles, the patellar tendon tension with the retinaculum intact was 8.9% less than the tension with the retinaculum removed. However, the de-

6 2047 crease in patellar tendon tension varied across the tested knee flexion angles, and significance was achieved at only the extremes of the range of motion tested (0 and 60 ). Retinaculum removal had the largest influence on the transmission of forces to the patellar tendon at 0, where a 16.6% increase in patellar tendon tension was measured. We postulated that the increase in the load-sharing function of the peripatellar retinaculum with the knee extended could be the result of the screw home mechanism of the knee complex. For example, it is well-established that the tibia externally rotates relative to the femur during knee extension The axial rotation of the tibia as the knee extends is thought to be the result of the shape of the medial femoral condyle, the passive tension in the anterior cruciate ligament, and the lateral pull of the quadriceps muscle 27. In their study of the influence of tibial rotation on strain in the peripatellar retinaculum, Lee et al. 21 demonstrated that the largest increases in strain occurred with tibial external rotation at 0 of flexion. Thus, it is conceivable that increased passive tension of the retinaculum as a result of tibial rotation could increase the load-sharing behavior of this structure. Additional strain in the peripatellar retinaculum with the knee extended also could result from the need for increased frontal plane stability of the patella as the osseous support afforded by the trochlear groove is minimal in this position 16,28. Interestingly, there were no significant differences in patellar tendon tension between the retinaculum-intact and retinaculum-removed conditions at 20 or 40 of flexion. Perhaps this was due to the positioning of the patella within the deeper portion of the trochlear groove, thereby minimizing the role of the peripatellar retinaculum as a frontal plane stabilizer of the patella. In addition, as the knee flexes, the tibia internally rotates to a more neutral position 27. Both of these events could result in a decrease in the passive tension within the retinaculum, thereby decreasing load-sharing. At 60 of flexion, a 9.6% increase in patellar tendon tension was observed in the retinaculum-removed condition. We hypothesized that, with greater knee flexion angles, there is a gradual increase in the passive tension within the peripatellar retinaculum, which may augment the load-sharing function of this structure. This probably occurs as a result of the retinaculum sharing some of the passive tension created by the lengthening of the extensor mechanism Although patellar tendon tension was not quantified at knee flexion angles of >60 in this investigation, we believe that this trend toward increased load-sharing would continue with increasing knee flexion angles. When considering the anatomy of the peripatellar retinaculum, it is easy to visualize how quadriceps forces may be distributed to other structures rather than just the patellar tendon (Fig. 4). The superficial fibers of the lateral retinaculum originate from the iliotibial band and the vastus lateralis fascia and insert into the lateral margin of the patella and the patellar tendon. Similarly, the superficial fibers of the medial retinaculum originate from the vastus medialis and the sartorius muscles and attach to the patella and the patellar tendon medially 14. The deep layer of the lateral retinaculum consists of several structures, including the transverse and lateral patellofemoral ligaments as well as the patellotibial band, while the deep layer of the medial retinaculum is composed of the medial patellofemoral, patellomeniscal, and medial patellotibial ligaments 13. These structures subsequently connect the patella to the iliotibial tract, the tibia, and the femur. Given this complex anatomic relationship, it is not surprising that the peripatellar retinaculum would function as a load-transmitting structure within the extensor mechanism. A limitation of this study was that submaximal muscle forces were employed during testing. This was done to maintain the integrity of the cadaver tissue. Although 276 N may not represent the force generated by the quadriceps during activities of daily living (e.g., walking), this load did allow adequate comparison of the conditions under study. The fact that significant differences were detected in patellar tendon tension under a submaximal loading condition may imply that the trends observed in the current investigation will be magnified when the retinaculum is under greater tension with higher quadriceps forces. Future studies should address the effects of increased loads on the force distribution within the extensor mechanism as a function of knee position. Our results have clinical implications in that any compromise of the peripatellar retinaculum may impair its loadsharing capability and increase the forces experienced by the patellar tendon. For example, in lateral patellar dislocation, the medial retinaculum is frequently partially or completely torn 32,33. Similarly, the lateral peripatellar retinaculum is commonly surgically disrupted in an effort to correct abnormal tracking of the patella 34. Given that the patellofemoral joint reaction force is the result of the quadriceps force vector and the patellar tendon vector, any increase in patellar tendon tension would translate into greater joint compression. Christopher M. Powers, PhD, PT Yu-Jen Chen, MS, PT Shawn Farrokhi, DPT Musculoskeletal Biomechanics Research Laboratory, Department of Biokinesiology and Physical Therapy, University of Southern California, 1540 Alcazar Street, CHP 155, Los Angeles, CA address for C.M. Powers: powers@usc.edu Thay Q. Lee, PhD Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System, 5901 East 7th Street, Long Beach, CA In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from the Whitaker Foundation. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated. doi: /jbjs.e.00929

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