Instability is reported to account for around 20% of early TKR revisions. The concept of restoring the “Envelope of Laxity” (EoL) mandates a balanced knee through a continuous arc of functional movement. We therefore hypothesised that a single radius (SR) design should confer this stability since it has been proposed that the SR promotes normal medial collateral ligament (MCL) function with isometric stability throughout the full arc of motion.

Our aim was to characterise the EoL and stability offered by a SR cruciate retaining (CR)-TKR, which maintains a SR from 10–110° flexion. This was compared with that of the native knee throughout the arc of flexion in terms of anterior, varus/valgus and internal/ external laxity to assess whether a SR CR-TKR design can mimic normal knee joint kinematics and stability.

Eight fresh frozen cadaveric lower limbs were physiologically loaded on a custom jig. The operating surgeon performed anterior drawer, varus/ valgus and internal/external rotation tests to determine ‘maximum’ displacements in 1) native knee and 2) single radius CR-TKR (Stryker Triathlon) at 0°, 30°, 60°, 90° and 110° flexion. Displacements were recorded using computer navigation. Significance was determined by linear modelling (p≤0.05).

The key finding of this work was that the EoL offered by the SR CR-TKR was largely equivalent to that of the native knee from 0–110°. The EoL increased significantly with flexion angle for both native and replaced knees. Overall, after TKR anterior laxity was comparable with the native knee, whilst total varus-valgus and internal-external rotational laxities reduced by only 1°. However, separated varus and valgus laxities at 110° significantly increased after TKR as did anterior laxity at 30° flexion.

In conclusion, the overall EoL offered by the SR CR-TKR is comparable to that of the native knee. In the absence of soft tissue deficiency, the implant appears to offer reliable and reproducible stability throughout the functional range of movement, with exception of anterior laxity at 30° and varus and valgus laxity when the knee approaches high flexion. These shortcomings should offer scope for future work.

Pre-operative variables are increasingly being used to determine eligibility for total knee replacement (TKR). This study was undertaken to evaluate the relationships, interactions and predictive capacity of variables available pre- and post-operatively on patient satisfaction following TKR. Using nationally collected patient reported outcome measures and data from the National Joint Registry for England and Wales, we identified 22 798 patients who underwent TKR for osteoarthritis between August 2008 and September 2010. The ability of specific covariates to predict satisfaction was assessed using ordinal logistic regression and structural equational modelling. Only 4959 (22%) of 22 278 patients rated the results of their TKR as ‘excellent’, despite the majority (71%, n = 15 882) perceiving their knee symptoms to be much improved. The strongest predictors of satisfaction were post-operative variables. Satisfaction was significantly and positively related to the perception of symptom improvement (operative success) and the post-operative EuroQol-5D score. While also significant within the models pre-operative variables were less important and had a minimal influence upon post-operative satisfaction. The most robust predictions of satisfaction occurred only when both pre- and post-operative variables were considered together. These findings question the appropriateness of restricting access to care based on arbitrary pre-operative thresholds as these factors have little bearing on post-operative satisfaction.

Cite this article: Bone Joint J 2013;95-B:1359–65.

Introduction

Renal homeostasis has been shown to influence mortality after hip fractures; this is true for patients with both chronic kidney disease, and those who develop acute renal dysfunction after surgery. We have examined the influence of impaired renal function upon mortality and length of stay. We investigated this relationship through accurate mathematical modelling of available biochemistry data on a cohort of hip fracture patients.

Purpose

To determine how patient characteristics; health status and pre-operative knee function effect patient reported satisfaction and operative success following total knee replacement (TKR). We hypothesised that a number of patient factors would influence the reported levels of symptomatic improvement (success) and satisfaction and that these two outcomes would represent different aspects of the patient's perception of outcome.