Over the last two decades, anatomic anterior cruciate ligament (ACL) reconstructions have gained popularity, while the use of extraarticular reconstructions has decreased. However, the biomechanical rationale behind the lateral extraarticular sling has not been adequately studied. By understanding its effect on knee stability, it may be possible to identify specific situations in which lateral extraarticular tenodesis may be advantageous. The primary objective of this study was to quantify the ability of a lateral extraarticular sling to restore native kinematics to the ACL deficient knee, with and without combined intraarticular anatomic ACL reconstruction. Additionally, we aimed to characterise the isometry of four possible femoral tunnel positions for the lateral extraarticular sling. Eight fresh frozen hip-to-toe cadavers were used in this study. Navigated Lachman and mechanised pivot shift examinations were performed on ACL itact and deficient knees. Three reconstruction strategies were evaluated: Single bundle anatomic intraarticular ACL reconstruction, Lateral extraarticular sling, Combined intraarticular ACL reconstruction and lateral extraarticular sling. After all stability tests were completed, we quantified the isometry of four possible femoral tunnel positions for the lateral extraarticular sling using the Surgetics navigation system. A single tibial tunnel position was identified and digitised over Gerdy's tubercle. Four possible graft positions were identified on the lateral femoral condyle: the top of the lateral collateral ligament (LCL); the top of the septum; the ideal tunnel position, as defined by the navigation system's own algorithm; and the actual tunnel position used during testing, described in the literature as the intersection of the linear projections of the LCL and the septum over the lateral femoral condyle. For each of the four tunnel positions, the knee was cycled from 0 to 90® of flexion and fiber length was recorded at 30® intervals, therefore quantiying the magnitude of anisometry for each tunnel position. Stability testing: Sectioning of the ACL resulted in an increase in Lachman (15mm, p = 0.01) and mechanised pivot shift examination (6.75mm, p = 0.04) in all specimens compared with the intact knee. Anatomic intraarticular ACL reconstruction restored the Lachman (6.7mm, p = 3.76) and pivot shift (−3.5mm, p = 0.85) to the intact state. With lateral extraarticular sling alone, there was a trend towards increased anterior translation with the Lachman test (9.2mm, p = 0.50). This reconstruction restored the pivot shift to the intact state. (1.25mm, p = 0.73). Combined intraarticular and extraarticular reconstruction restored the Lachman (6.2mm, p = 2.11) and pivot shift (−3.75mm, p = 0.41) to the intact state. There was no significant difference between intraarticular alone and combined intraarticular and extraarticular reconstruction. (p = 1.88) Isometry: The ideal tunnel position calculated by the navigation system was identified over the lateral femoral condyle, beneath the mid-portion of the LCL. The anisometry for the ideal tunnel position was significantly lower (5.9mm; SD = 1.8mm; P<0.05) than the anisometry of the actual graft position (14.9mm; SD = 4mm), the top of the LCL (13.9mm; SD = 4.3mm) and the top of the septum (12mm; SD = 2.4mm). In the isolated acute ACL deficient knee, the addition of a lateral extraarticular sling to anatomic intraarticular ACL reconstruction provides little biomechanical advantage and is not routinely recommended. Isolated lateral extraarticular sling does control the pivot shift, and may be an option in the revision setting or in the lower demand patient with functional instability. Additionally, the location of the femoral tunnel traditionally used results in a significantly more anisometric graft than the navigation's system mathematical ideal location. However, the location of this ideal tunnel placement lies beneath mid-portion of the fibers of the LCL, which would not be clinically feasible.
The aims of this study were: 1) to quantitatively analyse the amount of knee extension that is achieved with +2mm incremental increases in the amount of distal femoral bone that is resected during TKA in the setting of a flexion contracture, 2) to quantify the amount of coronal plane laxity that occurs with each 2mm increase in the amount of distal femur resected. In the setting of a soft tissue flexion contracture, we hypothesized that although resecting more distal femur will reliably improve maximal knee extension, it will ultimately lead to increased varus and/or valgus laxity throughout mid-flexion. Seven fresh-frozen cadaver legs from hip-to-toe underwent TKA with a posterior stabilized implant using a measured resection technique with computer navigation system equipped with a robotic cutting-guide, in this IRB approved, controlled laboratory study. After the initial tibial and femoral resections were performed, the posterior joint capsule was sutured (imbricated) through the joint space under direct visualization until a 10° flexion contracture was obtained with the trial components in place, as confirmed by computer navigation. Two distal femoral recuts of +2mm each where then subsequently made and after the remaining femoral cuts were made, the trail implants were reinserted. The navigation system was used to measure overall coronal plane laxity by measuring the mechanical alignment angle at maximum extension, 30°, 60° and 90° of flexion, when applying a standardized varus/valgus load of 9.8 [Nm] across the knee using a 4kg spring-load located at 25cm distal to the knee joint line.(Figure 1) Coronal plane laxity was defined as the absolute difference (in °) between the mean mechanical alignment angle obtained from applying a standardized varus and valgus stress at 0°, 30, 60° and 90°. Each measurement was performed three separate times and averaged. The maximal extension angle achieved following each 2mm distal recut was also recorded. Two-tailed student's t-tests were performed to analyze whether there was difference in the mean laxity at each angle and if there was a significant improvement in maximal extension with each recut. P-values < 0.05 were considered significant.Aims/Hypothesis
Methods
Unicompartmental knee replacement components have gained favor because they replace only the most damaged areas of articular cartilage and the less invasive operation results in a faster patient recovery than traditional TKR. Additionally, they can provide a solution when a full TKR is not yet needed. However, the wear magnitude of such implants is not well understood, primarily due the variation in design and the difficulty of testing them in knee simulators designed to test full TKRs. Modern innovative partial cartilage replacement knee components which are typically even smaller and more bone conservative than unicompartmental implants, are even less common in testing with added challenges. This study investigates the fatigue characteristics of partial cartilage replacement knee components, and the wear of the UHMWPE bearing of a new, truly less invasive unicompartmental design by Arthrex Inc./Florida. Fatigue testing was performed on MTS 858 MiniBionix machines. Two 12mm diameter UHMWPE tibial components were cemented into jigs at 0° posterior slope and were axially loaded at 2Hz for 10 million cycles (Mc) with a sinusoidal profile peaking at 60% of 8 average human bodyweights (3800N) and a load ratio R of 0.1. Two femoral components were tested with the same load profile at 10Hz for 10 million loading cycles (Mc). The femoral components were mounted at 15° flexion and only the anterior half of the implant was supported, replicating a worst-case scenario where fixation had failed on the posterior half of the implant. This resulted in a large bending moment when force was applied that would fatigue the femoral implant. Following the fatigue test, two full wear simulation tests were conducted on four 12mm and four 20mm unicompartmental components on a four-station Instron-Stanmore force-control knee simulator. The spring-based system to simulate soft-tissue restraining forces and torques was adapted to operate the machine in a displacement control mode to achieve the motions of the medial compartment based on ISO 14243-3. The specimens were lubricated with bovine serum (20g/L protein, 37°C) and the simulator was operated at 1Hz. Liquid absorption was corrected through passive-soak-control bearing inserts. The tibial specimens were cleaned and weighed at standard intervals with the usual ISO test protocols. After 10Mc of fatigue testing, both tibial components had deformed by some flattening out but were able to sustain the full load without failure and displayed average stiffness (over the whole 10Mc) of 27,600±1,180 N/mm. Neither partially supported femoral component failed, and the femorals displayed average stiffness (over 10Mc) of 37,500 ±3,280N/mm. After 5Mc of wear testing, the 12mm tibial components displayed a wear rate of 4.56±1.45mg/Mc while the larger 20mm size wore at a lower 2.80±0.39mg/Mc. The results from the fatigue test suggest that this unicompartmental cartilage replacement design will not fail under simple axial loading, even under the extreme case where the tibial implant is receiving the entire share of the load, and the femoral component is only partially supported. In the clinical application, of course some load-sharing with the native unworn cartilage would occur, reducing the stresses on the implant. The results from the wear test showed very low wear for tibial components of this design, lower than many successful TKRs. The larger size tibial components wore less likely due to reduced contact stress. Based on the results of this test, an implant of this type could be a viable option prior to TKR.
