Assessing And Quantifying Instability In Revision Total Knee Arthroplasty

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Margaret Curtis
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1 Assessing And Quantifying Instability In Revision Total Knee Arthroplasty David F. Hamilton, PHD, Richard Burnett, FRCS (Ed), James T. Patton, FRCS (Ed), Colin R. Howie, FRCS (Ed), Hamish Simpson, DPhil. University of Edinburgh, Edinburgh, United Kingdom. Disclosures: D.F. Hamilton: None. R. Burnett: 2; Stryker. J.T. Patton: 2; Stryker. C.R. Howie: None. H. Simpson: 5; Stryker, Pfizer Astrazeneca, DePuy. Introduction: Instability is a common cause of poor outcome following total knee arthroplasty (TKA), and accounts for around 20% of revision surgery. It is often difficult to establish a diagnosis in these cases.in the absence of definitive diagnostic criteria, surgical diagnosis is based on patient symptoms, clinical and radiological assessment. Knee instability manifests as a failure to maintain normal knee function, with patients typically reporting the knee giving-way during load bearing activity on a flexed knee. Knee bracing and exercise rehabilitation may be helpful in reducing the symptoms in some patients with mild or moderate instability. In most cases surgical intervention is required. There are few studies that report the outcomes of revision arthroplasty for instability. We hypothesised that performing a power output assessment would highlight and potentially quantify the patient s instability symptoms, as it involves application of force through a flexed knee. The aim of this study was to assess whether a standardised forceful closed-chain leg extension activity could be used to assess instability of a primary total knee replacement. Methods: 25 consecutive patients undergoing revision TKA for a specific diagnosis of knee instability (following a primary TKA) were identified over a two year period ( ).Patients were recruited from a single high volume orthopaedic teaching hospital. There were 11 men and 14 women with a mean age of (8.73) years.knee instability was clinically diagnosed by the surgical team by assessing laxity of the knee in all planes and both flexion and extension, in conjunction with patient reported symptoms. All surgical procedures were revision of a primary total knee replacement. Revision implant choice was at the discretion of the surgeon depending on the bone loss and the level of constraint required in each case. 14 patients received a semi-constrained implant (Triathlon Total Stabilizer, Stryker) and 11 a constrained implant (9 patients an Endo-Modell Rotational Knee Prosthesis, Link; and 2 patients a Modular Rotating Hinge, Stryker). Surgery was performed by multiple consultant orthopaedic surgeons and their supervised trainees. All patients received identical routine post-operative care. Rehabilitation encompassed mobilisation on the day of surgery and functional inpatient physiotherapy as per the unit protocol. All patients were assessed pre-operatively at the time of the pre-admission clinic (approx. 2 weeks prior to surgery), and then at the 6 week and 26 weeks post-operative clinical review. A control group of 183 primary total knee replacements was evaluated for comparison. This cohort was a randomised controlled trial performed in our unit investigating lower limb power output following primary knee arthroplasty for osteoarthritis. This group was chosen as it had corresponding poweroutput assessments at the same time points.

2 Forceful knee extension was assessed with a Nottingham rig, depressing a pedal a fixed distance from a bent knee position; simulating the situation where giving way is reported. Knee instability was categorised as inability to maintain constant force through depression of the pedal. We recorded this as a reduction in revolutions per minute (rpm) generated in a single test, assessed by direct measurement of the output graphs. Patients were assessed pre-op and then at 6 and 26 weeks post-op. A historic control group of 183 primary TKAs with equivalent assessments was evaluated for comparison.data were analysed using Minitab (16) software. Change in continuous variables over time was assess by repeated measures analysis of variance (ANOVA) and comparative analysis between revision and control groups was assessed by independent samples t-test or by chi-squared test. Significance was accepted at p = Results: Inability to provide a continuous force through depression of the Nottingham Rig was reflected by an initial increase in flywheel speed, subsequent reduction in speed, and secondary recovery of flywheel speed within a single depression of the footplate by the patient (figure 1). All 25 cases of revision of primary TKA for instability exhibited this distinctive double-push pattern prior to surgery. The mean pre-operative mid-push reduction in flywheel speed was 55 (33.2) rpm which represented a 64% mid-push reduction in flywheel speed from the maximal value achieved. Following revision total knee arthroplasty, none of the 25 patients exhibited this output pattern; mid-push reduction in speed was 0 rpm in all cases (figure 2). Change between pre- and post -op assessment was highly significant (ANOVA, p = <0.001). Post-hoc analysis found no difference in pre-operative mid-push reduction in flywheel speed (Independent t-test, p = 0.81), or in graph output pattern between the differing implants used (figure 3). The control group of primary TKAs had the same gender split and mean age as the revision surgery group. No patients in the control group of primary total knee replacements exhibited this output pattern at any of the time points assessed, the between group difference therefore also being significant (Chi Sq. p = <0.001). Discussion: All patients undergoing revision TKA for instability demonstrated a distinctive forceful knee extension output pattern pre-op that was corrected with surgery in every case. No difference in output pattern was seen between patients requiring differing levels of constraint (hinge vs. total stabiliser), suggesting that surgical correction of the mechanically unstable knee is the primary factor in eradicating symptoms. A correct diagnosis of instability of the primary implant is of paramount importance to outcomes following revision surgery. The patient s report of instability is not a diagnosis but a presenting complaint. The clinical exam is a key factor in determining the correct course of action, though this is subjective and dependent on the clinician s skills and experience. This simple tool potentially aids consistency in diagnosis. Significance: This study suggests that diagnostic criteria to assess the unstable primary total knee replacements may be developed.

5 ORS 2015 Annual Meeting Poster No: 1783

ATTUNE Knee System: Stability in Total Knee Replacement

ATTUNE Knee System: Stability in Total Knee Replacement Chadd Clary, PhD Staff Engineer DePuy Synthes Joint Reconstruction Young and active total knee replacement (TKR) patients demand a knee that feels