There is a renewed debate regarding the relative importance of (primarily varus-valgus) stability versus alignment in TKA. Some surgeons have posited that stability is of greater importance. Perhaps this is because unstable knees fail immediately whereas mal-aligned knees generally suffer late failure from wear, osteolysis and loosening. Or perhaps some surgeons find soft tissue techniques challenging. Clearly alignment and stability are both necessary for immediate function and long-term durability.

Ligament tensioners are as old as condylar knee arthroplasties. They first appeared when surgeons moved beyond hinged arthroplasties with a goal of melding anatomy and biomechanics- to re-establish stability and correct pathologic deformity. Early techniques stipulated that ligament releases should be performed first, before any bone cuts thus correcting deformity and restoring stability. Crude mechanical instruments were replaced by mechanical devices.

Acknowledging more exacting standards, our ability to hit the target of desired alignment and stability is limited unassisted. As more sophisticated devices have been introduced to help surgeons correct alignment we have not yet discovered the perfect mechanical, electronic, navigated or laser guided “tensioner”. We still struggle to divine the “best” alignment. The principle however endures, that integrating stability and alignment, if with nothing more than a “cognitive tensioner” is essential to optimal short and long-term arthroplasty function.

Stability after TKA is essential for knee function and patient satisfaction. Stability may be marginally more important even than alignment because “stability” means there will be ONE alignment, whereas INSTABILITY means there will be many alignments of the joint, usually the worst one for any loading pattern. Whereas alignment results from the orientation and size of implants, stability depends on all of these, plus soft tissue integrity and in many cases, surgical alteration. Ligament releases (and rarely reconstructions) will certainly be required if alignment is changed with the arthroplasty. Instability may be a subtle or flagrant problem.

The “Instabilities” are: i.

Varus- valgus

Varus-valgus instability is the prototype and while it may originate exclusively from the failure of soft tissue, knee alignment and dynamic forces outside the knee joint such as hip abductor dysfunction, scoliois and tibialis posterior rupture may be implicated. A comprehensive approach will be needed.

Flexion instability, most simply stated results from a flexion gap that exceeds the dimensions of the extension gap. It will result most commonly after surgery for the patient with a fixed flexion contracture whose knee extends fully because a relatively thin polyethylene insert has been selected. So-called “mid-flexion” instability (implying stability in extension and flexion) has not yet been thoroughly characterised.

Extension instability includes all failures of the extensor mechanism (rupture, maltracking and weakness) which are better characterised as “buckling” under a separate topic. Recurvatum has received little attention but can generate the most destructive forces leading to knee arthroplasty failure. In general begins as a compensatory mechanism for relative extensor weakness.

All treatment of the unstable TKA must characterise the mode(s) of failure above and correct the underlying cause. Surgical technique will be extremely important, followed eventually by implant selection.

In the treatment of ligament injuries there has been much interest in the restoration of the actual ligament anatomy, and the extent to which the original enthesis may be re-established. This study therefore seeks to uncover new information on ligament microstructure and its insertion into bone.

Five bovine medial collateral ligaments (MCL) and five ovine anterior cruciate ligaments (ACL) were used in this study. All ligaments were harvested with the femoral and tibial bony insertions still intact. The bone ends were clamped and the MCL stretched to about 10% strain while the ACL underwent a 90° twist. The entire ligament-bone system, under load, was fixed in 10% formalin solution for 12 hours, following which it was partially decalcified to facilitate microsectioning. Thin 30 ìm-thick sections of the ligament-bone interface and ligament midsubstance were obtained. Differential Interference Contrast (DIC) optical microscopy was used to image the ligament and bone microarchitecture in the prescribed states of strain.

Fibre crimp patterns were examined for the prescribed loading condition and showed distinct sections of fibre recruitment. Transverse micro-imaging of the ligament showed a significant variation in the sub-bundle cross-sectional area, ranging from 100ìm to 800 ìm. Those bundles closer to the central long axis of the ligament were numerous and small, while moving towards the periphery, they were large and singular. Both classifications of entheses, direct and indirect, were observed in the MCL insertions into the femur and tibia respectively. Of interest was the indirect insertion where the macro-level view of the near parallel attachment of fibres to bone via the periosteum was revealed, at the microscale, to involve a gradually increasing orthogonal insertion of fibres. This unique transition occurred closer to the joint line. In the ACL the anterior-medial (AM) and posterior-lateral (PL) bundles were easily discernable. All insertions into bone for the ACL were of the direct type. Fibres were thus seen to transition through the four zones of gradual mineralization to bone. However the manner in which the AM and PL bundles insert into bone, and the lateral soft tissue transition between these two bundles, revealed a structural complexity that we believe is biomechanically significant.

This ‘mechano-structural’ investigation, using novel imaging techniques, has provided new insights into the microstructure of the ligament bone system. The images presented from this study are aimed to aid new approaches for reconstruction, and provide a blue-print for the design of ligament-bone systems via tissue engineering.

Despite modern surgical techniques, reported rates of deep infection following Total Knee Replacement (TKR) persist between 1–2.5%. Coagulase-negative staphylococcus (CNS) has become the most common causative organism, and while growth of CNS is more indolent thanstaphylococcus aureus, it has a relatively higher minimum inhibitory concentration (MIC) against cephalosporins. Tissue concentrations of prophylactic antibiotics may fall below this level during TKR with conventional ‘systemic’ dosing.

Regional administration of prophylactic antibiotics via a foot vein following tourniquet inflation has been shown to provide tissue concentrations approximately 10 times higher than systemic dosing, however cannulation of a foot vein is difficult, time consuming, and may compromise sterility.

Intraosseous cannulation offers an alternative method of accessing the vascular system, and the aim of this study was to assess its effectiveness in administration of prophylactic antibiotics. 22 patients undergoing primary total knee arthroplasty were randomised into two groups. Group 1 received 1g of cephazolin systemically 10 minutes prior to tourniquet inflation. In Group 2 the EZ-IO tibial cannulation system was used, and 1g of cephazolin was administered intraosseously in 200ml of normal saline following tourniquet inflation and prior to skin incision. Subcutaneous fat and femoral bone samples were taken at set intervals during the procedure, and antibiotic concentrations measured using High Performance Liquid Chromatography (HPLC).

There were no significant differences in patient demographics, comorbidities, or physical parameters between groups. The overall mean tissue concentration of cephazolin in subcutaneous fat was 185.9μg/g in the intraosseous group and 10.6μg/g in the systemic group (p<0.01). The mean tissue concentration in bone was 129.9 μg/g in the intraosseous group and 11.4μg/g in the systemic group (p<0.01). These differences were consistent across all sample time points throughout the procedure. No complications occurred in either group.

Intraosseous regional administration can achieve tissue levels of antibiotic over an order of magnitude higher than systemic administration. Further work is required to determine if there is clinical benefit in preventing infection, particularly against CNS. This novel mode of drug administration may also have other applications, allowing ‘surgical site delivery’ of medication while minimising systemic side effects.