Breck R. Lord

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.

The Envelope of Laxity of the Pivot Shift Test

The pivot shift is a dynamic test of knee laxity which correlates with subjective sensations of knee instability. As the knee flexes from full extension, the tibia subluxes, both in anterior translation and internal rotation, so that the lateral femoral condyle moves ‘downhill’ to the posterior edge of the tibial plateau under the influence of the compressive joint load. With further knee flexion, the tension in the iliotibial tract eventually overcomes the load which has maintained the subluxation and then the tibia is suddenly reduced to its anatomical articulation. Thus, the envelope of laxity of the pivot shift shows a pattern of simultaneous gradual pathological anterior translation and internal rotation, occurring over approximately 35° of knee flexion, followed by a relatively sudden reduction, which is a falling back posteriorly and externally to the anatomical position. It is desirable to measure both tibial translations and rotations to understand each injured knee, because differing patterns of injury may explain the wide range of tibiofemoral movements, such as the relative amount of tibial translation versus rotation, that have been reported during the pivot shift.

Assessment of pose repeatability and specimen repositioning of a robotic joint testing platform

This paper describes the quantitative assessment of a robotic testing platform, consisting of an industrial robot and a universal force-moment sensor, via the design of fixtures used to hold the tibia and femur of cadaveric knees. This platform was used to study the contributions of different soft tissues and the ability of implants and reconstruction surgeries to restore normal joint functions, in previously published literature. To compare different conditions of human joints, it is essential to reposition specimens with high precision after they have been removed for a surgical procedure. Methods and experiments carried out to determine the pose repeatability and measure errors in repositioning specimens are presented. This was achieved using an optical tracking system (fusion Track 500, Atracsys Switzerland) to measure the position and orientation of bespoke rigid body markers attached to the tibial and femoral pots after removing and reinstalling them inside the rigs. The pose repeatability was then evaluated by controlling the robotic platform to move a knee joint repeatedly to/from a given pose while tracking the position and orientation of a rigid body marker attached to the tibial fixture. The results showed that the proposed design ensured a high repeatability in repositioning the pots with standard deviations for the computed distance and angle between the pots at both ends of the joint equal to 0.1mm, 0.01mm, 0.13° and 0.03° for the tibial and femoral fixtures respectively. Therefore, it is possible to remove and re-setup a joint with high precision. The results also showed that the errors in repositioning the robotic platform (that is: specimen path repeatability) were 0.11mm and 0.12°, respectively.

Biomechanical Evaluation of Human Allograft Compression in Anterior Cruciate Ligament Reconstruction

Introduction: A common problem encountered during ACL reconstruction is asymmetry of proximal-distal graft diameter leading to tunnel upsizing and potential graft-tunnel mismatch. Human allografts are often oedematous, compounding this issue in the context of multi-ligament reconstructions. Tunnel upsizing reduces bone stock, increases the complexity of multi-bundle surgery and may compromise graft-osseous integration if cortical suspensory fixation is used. Graft compression provides uniform size, allowing easy passage into a smaller tunnel, potentially improving the ‘press-fit’ graft-osseous interaction whilst preserving bone stock. To our knowledge, no biomechanical evaluation of this increasing popular technique has been reported. Hypotheses: Graft compression would not cause any significant changes in the biomechanical properties of human allograft tendon that would be detrimental to the function of an ACL reconstruction. Compressed Bioclense® allograft will increase in size when soaked in Ringer’s solution at 36° improving the ‘press-fit’ within the bone socket, decreasing micro-motion at the graft-osseous interface following ACL reconstruction. Method: In-vitro laboratory study. Sixteen samples of Bioclense® treated peroneus longus allograft were quadrupled into GraftLink constructs randomly divided into control and compressed groups. Cross-sectional area (CSA) was determined using alginate moulds and specimens immersed, under tension, in Ringer’s solution at 36.5°. CSA was measured at 8 hours. A further 32 samples were randomised and evaluated under cyclic loading of 70N-220N (1020 cycles) followed by test to failure. A further 30 samples were quadrupled into GraftLink constructs and mounted within porcine femurs using suspensory fixation. High resolution videometer recorded motion at the graft-osseous interface under the same cyclic loading protocol. An independent samples t-test was used to compare changes in CSA whilst a one-way ANOVA was used for biomechanical end points. Results: CSA increased by 1.2 ± 0.04% and 16 ± 0.07% in control and compressed groups during joint simulation (P<0.05). Cyclic creep was 0.62 ± 1.22mm and 1.75 ± 0.97 (P>0.05), the Young’s moduli were 617 ± 172 MPa and 708 ± 219 MPa (P>0.05) and ultimate tensile strength 85.2 ± 27.4 MPa and 89 ± 25.3 MPa (P>0.05) for the control and compressed groups respectively. Initial samples (n=4) show amplitude of cyclic motion of control and compressed ACL grafts in situ were 2.1mm (±0.6) and 1.9mm (±0.7) for control and compressed groups respectively. Conclusions: The process of graft compression does not have any detrimental effects upon Bioclense® treated allograft tendons. Following graft compression, these tendons significantly increase in size during intra-articular simulation promoting a ‘press-fit’ within the bone socket. Graft compression may significantly decrease micro-motion at the graft-osseous interface with further testing. Clinical Relevance: Graft compression is a biomechanically safe adjunct to ACL reconstruction when using Bioclense® treated allograft, aiding surgical technique and preserving bone stock.