This in-vitro study finds which hip joint soft tissues act as primary and secondary passive internal and external rotation restraints so that informed decisions can be made about which soft tissues should be preserved or repaired during hip surgery. The capsular ligaments provide primary hip rotation restraint through a complete hip range of motion protecting the labrum from impingement. The labrum and ligamentum teres only provided secondary stability in a limited number of positions. Within the capsule, the iliofemoral lateral arm and ischiofemoral ligaments were primary restraints in two-thirds of the positions tested and so preservation/repair of these tissues should be a priority to prevent excessive hip rotation and subsequent impingement/instability for both the native hip and after hip arthroplasty

Introduction. There is little information available to surgeons regarding how the lateral soft-tissue structures prevent instability in knees implanted with total knee arthroplasty (TKA). The aim of this study was to quantify the lateral soft-tissue contributions to stability following cruciate retaining (CR) TKA. Methods. Nine cadaveric knees with CR TKA implants (PFC Sigma; DePuy Synthes Joint Reconstruction) were tested in a robotic system (Fig. 1) at full extension, 30°, 60°, and 90° flexion angles. ±90 N anterior-posterior force, ±8 Nm varus-valgus and ±5 Nm internal-external torque were applied at each flexion angle. The anterolateral structures (ALS, including the iliotibial band, anterolateral ligament and anterolateral capsule), the lateral collateral ligament (LCL), the popliteus tendon complex (Pop T) and the posterior cruciate ligament (PCL) were then sequentially transected. After each transection the kinematics obtained from the original loads were replayed, and the decrease in force / moment equated to the relative contributions of each soft-tissue to stabilising the applied loads. Results. In the CR TKA knee, the LCL was found to be the primary restraint to varus laxity (Fig. 2, an average 56% across all flexion angles), and was significant in internal-external rotational stability (28% and 26% respectively) and anterior drawer (16%). The ALS restrained 25% of internal rotation (Fig. 3), whilst the PCL was significant in posterior drawer only at 60° and 90° flexion. The Pop T was not found to be significant in any tests. Conclusion. This study has for the first time delineated the relative contributions of lateral structures to stability in the implanted knee. It was confirmed that the LCL is the major lateral structure in CR TKA stability throughout the arc of flexion. In the event of LCL deficiency, stability of the knee may only be restored by either changing to a more constrained implant or performing a reconstruction of the ligament. Furthermore, care should be taken when releasing the LCL to correct a valgus deformity as it may result in a combined rotational laxity pattern that cannot be overcome by the other passive lateral structures or the PCL. For figures, please contact authors directly

INTRODUCTION. Understanding the biomechanics of the anatomical knee is vital to innovations in implant design and surgical procedures. The anterior – posterior (AP) laxity is of particular importance in terms of functional outcomes. Most of the data on stability has been obtained on the unloaded knee, which does not relate to functional knee behavior. However, some studies have shown that AP laxity decreases under compression (1) (2). This implies that while the ligaments are the primary stabilizers under low loads, other mechanisms come into play in the loaded knee. It is hypothesized this decreased laxity with compressive loads is due to the following: the meniscus, which will restrain the femur in all directions; the cartilage, which will require energy as the femur displaces across the tibial surface in a plowing fashion; and the upwards slope of the anterior medial tibial plateau, which stabilizes the knee by a gravity mechanism. It is also hypothesized that the ACL will be the primary restraint for anterior tibial translation. METHODS. A test rig was designed where shear and compressive forces could be applied and the AP and vertical displacements measured (Figure 1). The AP motion was controlled by the air bearings and motor, allowing for the accurate application of the shear force. Position and force data were measured using load cells, potentiometers, and a linear variable differential transducer. Five knee specimens less than 60 years old and without osteoarthritis (OA), were evaluated at compressive loads of 0, 250, 500, 750 N, with the knee at 15° flexion. Three cycles of shear force at ±100 N constituted a test. The intact knee was tested, followed by testing after each of the following resections: LCL, MCL, PCL, ACL, medial meniscus, and lateral meniscus. RESULTS. The average displacement of the tibia without load was 6.17 mm anterior and −4.92 mm posterior. Under load the posterior translation of the tibia was reduced essentially to zero. After ACL resection, the anterior tibial displacement increased substantially, with a further increase after medial meniscus resection. Cartilage deformation had a minimal effect. DISCUSSION. The hypotheses that the ACL and the upwards tibial slope would provide stability under load were validated. The ACL was essential under all load conditions because the posterior tibial surface was flat (figure 2). The medial meniscus provided vertical stability, as a space buffer (figure 3), and in two specimens under load it provided the same restraint as the ACL (figure 2). The experiment was limited by lack of muscle action, the number of specimens, and a single flexion angle. SIGNIFICANCE. The test rig and methodology had capabilities exceeding those of previous work in determining the mechanisms of AP knee stability under load due to its frictionless air bearings. The results have application ranging from sports medicine to total knee design. The stabilizing effect of the tibial slope seen here validates tibial osteotomies for improved stability. The importance of reproducing ACL function in total knee design is emphasized. For figures/tables, please contact authors directly.

Controversies about the management of injuries to the soft tissue structures of the posteromedial corner of the knee and the contribution of such peripheral structures on rotational stability of the knee are of increasing interest and currently remain inadequately characterised. The posterior oblique ligament (POL) is a fibrous extension off the distal aspect of the semimembranosus that blends with and reinforces the posteromedial aspect of the joint capsule. The POL is reported to be a primary restraint to internal rotation and a secondary restraint to valgus translation and external rotation. Although its role as a static stabiliser to the medial knee has been previously described, the effect of the posterior oblique ligament (POL) injuries on tibiofemoral stability during Lachman and pivot shift examination in the setting of ACL injury is unknown. The objective of this study was to quantify the magnitude of tibiofemoral translation during the Lachman and pivot shift tests after serial sectioning of the ACL and POL. Eight knees were used for this study. Ligamentous constraints were sequentially sectioned in the following order: ACL first, followed by the POL. Navigated mechanised pivot shift and Lachman examinations were performed before and after each structure was sectioned, and tibiofemoral translation was recorded. Lachman test: There was a mean 6.0 mm of lateral compartment translation in the intact knee (SD = 3.3 mm). After sectioning the ACL, translation increased to 13.8 mm (SD = 4.6; P<0.05). There was a nonsignificant 0.7 mm increase in translation after sectioning the POL (mean = 14.5 mm; SD = 3.9 P>0.05). Mechanised pivot shift: Mean lateral compartment translation in the intact knee was −1.2 mm (SD = 3.2 mm). Sectioning the ACL caused an increase in anterior tibial translation (mean = 6.7 mm; SD = 3.0 mm; P<0.05). No significant change in translation was seen after sectioning the POL (mean = 7.0 mm, SD = 4.0 mm; P>0.05). Sectioning the POL did not significantly alter tibiofemoral translation in the ACL deficient knee during the Lachman and pivot shift tests. This study brings into question whether injuries to the POL require reconstruction in conjunction with ACL reconstruction. More studies are needed to further characterise the role of the injured POL in knee stability and its clinical relevance in the ACL deficient and reconstructed knee

Controversies about the management of injuries to the soft tissue structures of the posteromedial corner of the knee and the contribution of such peripheral structures on rotational stability of the knee are of increasing interest and currently remain inadequately characterised. The posterior oblique ligament (POL) is a fibrous extension off the distal aspect of the semimembranosus that blends with and reinforces the posteromedial aspect of the joint capsule. The POL is reported to be a primary restraint to internal rotation and a secondary restraint to valgus translation and external rotation. Although its role as a static stabiliser to the medial knee has been previously described, the effect of the posterior oblique ligament (POL) injuries on tibiofemoral stability during Lachman and pivot shift examination in the setting of ACL injury is unknown. The objective of this study was to quantify the magnitude of tibiofemoral translation during the Lachman and pivot shift tests after serial sectioning of the ACL and POL. Eight knees were used for this study. Ligamentous constraints were sequentially sectioned in the following order: ACL first, followed by the POL. Navigated mechanised pivot shift and Lachman examinations were performed before and after each structure was sectioned, and tibiofemoral translation was recorded. Lachman test: There was a mean 6.0 mm of lateral compartment translation in the intact knee (SD = 3.3 mm). After sectioning the ACL, translation increased to 13.8 mm (SD = 4.6; P<0.05). There was a nonsignificant 0.7 mm increase in translation after sectioning the POL (mean = 14.5 mm; SD = 3.9 P>0.05). Mechanised pivot shift: Mean lateral compartment translation in the intact knee was −1.2 mm (SD = 3.2 mm). Sectioning the ACL caused an increase in anterior tibial translation (mean = 6.7 mm; SD = 3.0 mm; P<0.05). No significant change in translation was seen after sectioning the POL (mean = 7.0 mm, SD = 4.0 mm; P>0.05). Sectioning the POL did not significantly alter tibiofemoral translation in the ACL deficient knee during the Lachman and pivot shift tests. This study brings into question whether injuries to the POL require reconstruction in conjunction with ACL reconstruction. More studies are needed to further characterise the role of the injured POL in knee stability and its clinical relevance in the ACL deficient and reconstructed knee