Kiron K. Athwal

The Role of the Anterolateral Structures and the ACL in Controlling Laxity of the Intact and ACL-Deficient Knee

Background: Anterolateral rotatory instability (ALRI) may result from combined anterior cruciate ligament (ACL) and lateral extra-articular lesions, but the roles of the anterolateral structures remain controversial. Purpose: To determine the contribution of each anterolateral structure and the ACL in restraining simulated clinical laxity in both the intact and ACL-deficient knee. Study Design: Controlled laboratory study. Methods: A total of 16 knees were tested using a 6 degrees of freedom robot with a universal force-moment sensor. The system automatically defined the path of unloaded flexion/extension. At different flexion angles, anterior-posterior, internal-external, and internal rotational laxity in response to a simulated pivot shift were tested. Eight ACL-intact and 8 ACL-deficient knees were tested. The kinematics of the intact/deficient knee was replayed after transecting/resecting each structure of interest; therefore, the decrease in force/torque reflected the contribution of the transected/resected structure in restraining laxity. Data were analyzed using repeated-measures analyses of variance and paired t tests. Results: For anterior translation, the intact ACL was clearly the primary restraint. The iliotibial tract (ITT) resisted 31% ± 6% of the drawer force with the ACL cut at 30° of flexion; the anterolateral ligament (ALL) and anterolateral capsule resisted 4%. For internal rotation, the superficial layer of the ITT significantly restrained internal rotation at higher flexion angles: 56% ± 20% and 56% ± 16% at 90° for the ACL-intact and ACL-deficient groups, respectively. The deep layer of the ITT restrained internal rotation at lower flexion angles, with 26% ± 9% and 33% ± 12% at 30° for the ACL-intact and ACL-deficient groups, respectively. The other anterolateral structures provided no significant contribution. During the pivot-shift test, the ITT provided 72% ± 14% of the restraint at 45° for the ACL-deficient group. The ACL and other anterolateral structures made only a small contribution in restraining the pivot shift. Conclusion: The ALL and anterolateral capsule had a minor role in restraining internal rotation; the ITT was the primary restraint at 30° to 90° of flexion. Clinical Relevance: The ITT showed large contributions in restraining anterior subluxation of the lateral tibial plateau and tibial internal rotation, which constitute pathological laxity in ALRI. In cases with ALRI, an ITT injury should be suspected and kept in mind if an extra-articular procedure is performed.

Clinical biomechanics of instability related to total knee arthroplasty

BACKGROUND Tibiofemoral instability is a common reason for total knee arthroplasty failure, and may be attributed to soft tissue deficiency and incorrect ligament balancing. There are many different designs of implant with varying levels of constraint to overcome this instability; however there is little advice for surgeons to assess which is suitable for a specific patient, and soft tissue balance testing during arthroplasty is very subjective. METHOD The current theories on primary and secondary soft tissue restraints to anterior/posterior, varus/valgus, and internal/external rotational motion of the knee are discussed. The paper reviews biomechanics literature to evaluate instability in the intact and implanted knee. FINDINGS The paper highlights important intra- and extra-capsular structures in the knee and describes the techniques used by clinicians to assess instability perioperatively. In vitro cadaveric studies were found to be a very useful tool in comparing different implants and contributions of different soft tissues. INTERPRETATION In vitro cadaveric studies can be utilised in helping less experienced surgeons with soft tissue releases and determining the correct implant. For this to happen, more biomechanical studies must be done to show the impact of release sequences on implanted cadavers, as well as determining if increasingly constrained implants restore the stability of the knee to pre-deficient conditions.

Lack of evidence to support present medial release methods in total knee arthroplasty

PurposeThe aim of this review was to identify a reliable sequential medial release protocol for restoration of soft tissue balance in total knee arthroplasty of the varus osteoarthritic knee and to allow for improved intraoperative decision-making.MethodCurrent medial release sequences and applicability based upon pre-operative deformity have been reviewed. Furthermore, risks associated with over release, and the necessity of medial release, are discussed.ResultsThe different medial release sequences are discussed in relation to pre-operative deformity, along with potential complications associated with medial release. It was found that release sequences may include the deep and superficial components of the medial collateral ligament, the posteromedial capsule, the posterior oblique ligament, the pes anserinus (pes A), and tendons of the semimembranosus and medial gastrocnemius muscle. The sequences described were found to vary substantially between studies, and very few studies had systematically quantified the effect of each release on balance.ConclusionWhile medial release is the standard intraoperative mode of balancing, there is a lack of evidence to support current methods. The correct method for defining intraoperatively the sequence, extent and magnitude of releases required remains ill-defined. It could be argued that the classic extensive medial release may be unnecessary and may be associated with iatrogenic injury to the pes A and saphenous nerve, instability and abnormal knee kinematics. Minimal medial release may allow for improved soft tissue balancing leading ultimately to improved functional outcome.Level of evidenceV (expert opinion).

How Does Laxity After Single Radius Total Knee Arthroplasty Compare With the Native Knee

Patients with total knee arthroplasties (TKAs) continue to report dissatisfaction in functional outcome. Stability is a major factor contributing to functionality of TKAs. Implants with single‐radius (SR) femoral components are proposed to increase stability throughout the arc of flexion. Using computer navigation and loaded cadaveric legs, we characterized the “envelope of laxity” (EoL) offered by a SR cruciate retaining (CR)‐TKA compared with that of the native knee through the arc of flexion in terms of anterior drawer, varus/valgus stress, and internal/external rotation. In both the native knee and the TKA laxity increased with increasing knee flexion. Laxities measured in the three planes of motion were generally comparable between the native knee and TKA from 0° to 110° of flexion. Our results indicate that the SR CR‐TKA offers appropriate stability in the absence of soft tissue deficiency.

Isolated popliteus tendon injury does not lead to abnormal laxity in posterior-stabilised total knee arthroplasty.

AbstractPurpose The popliteus tendon is crucial to postero-lateral stability and prone to iatrogenic injury intra-operatively. Its role in the stability of the replaced knee remains contentious. The aim of this study was to use computer navigation to quantify the effect of popliteus sectioning on the ‘envelope of laxity’ (EoL) offered by a posterior-stabilised (PS) total knee arthroplasty (TKA) and compare with that of the native knee.MethodsLoaded cadaveric legs were mounted on a purpose built rig. EoL was measured in 3 degrees of freedom using computer navigation. Knees were subjectively stressed in varus/valgus, internal/external rotation and anterior draw. This was performed preoperatively, during TKA and after sectioning of the popliteus tendon. Real-time data were recorded at 0°, 30°, 60° and 90° of flexion as the operating surgeon stressed the knee in 3 degrees of freedom to its subjective endpoint. Mixed-effect modelling was used to quantify the effects of intervention on degree of laxity.ResultsIn all conditions, there was an increase in laxity with knee flexion. Insertion of a PS TKA resulted in increased constraint, particularly in rotation. Sectioning of the popliteus did not result in a significant increase in knee laxity to 90º of knee flexion. However, at deeper flexion angles, tendon sectioning overcame the constraints of the implant resulting in a significant increase in rotatory and varus/valgus laxity towards the native condition.ConclusionThese findings support the view that certain current designs of PS knee replacement can constrain the knee in flexion in the absence of postero-lateral deficiency. For this implant, isolated sectioning of the popliteus tendon did not substantially generate abnormal knee laxity.

Pre-Clinical Assessment of Total Knee Replacement Anterior-Posterior Constraint

Pre-clinical, bench-top assessment of Total Knee Replacements (TKR) can provide information about the inherent constraint provided by a TKR, which does not depend on the condition of the patient undergoing the arthroplasty. However little guidance is given by the ASTM standard on test configurations such as medial-lateral (M:L) loading distribution, flexion angle or restriction of secondary motions. Using a purpose built rig for a materials testing machine, four TKRs currently in widespread clinical use, including medial-pivot and symmetrical condyle types, were tested for anterior-posterior translational constraint. Compressive joint loads from 710 to 2000 N, and a range of medial-lateral (M:L) load distributions, from 70:30% to 30:70% M:L, were applied at different flexion angles with secondary motions unconstrained. It was found that TKA constraint was significantly less at 60 and 90° flexion than at 0°, whilst increasing the compressive joint load increased the force required to translate the tibia to limits of AP constraint at all flexion angles tested. Additionally when M:L load distribution was shifted medially, a coupled internal rotation was observed with anterior translation and external rotation with posterior translation. This paper includes some recommendations for future development of pre-clinical testing methods.

Lateral soft‐tissue structures contribute to cruciate‐retaining total knee arthroplasty stability

Little information is available to surgeons regarding how the lateral structures prevent instability in the replaced knee. The aim of this study was to quantify the lateral soft‐tissue contributions to stability following cruciate‐retaining total knee arthroplasty (CR TKA). Nine cadaveric knees were tested in a robotic system at full extension, 30°, 60°, and 90° flexion angles. In both native and CR implanted states, ±90 N anterior–posterior force, ±8 Nm varus–valgus, and ±5 Nm internal–external torque were applied. The anterolateral structures (ALS, including the iliotibial band), the lateral collateral ligament (LCL), the popliteus tendon complex (Pop T), and the posterior cruciate ligament (PCL) were transected and their relative contributions to stabilizing the applied loads were quantified. The LCL was found to be the primary restraint to varus laxity (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, while 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. Therefore, the LCL was confirmed as the major lateral structure in CR TKA stability throughout the arc of flexion and deficiency could present a complex rotational laxity that cannot be overcome by the other passive lateral structures or the PCL.