The identification and quantification of instability in a primary total knee replacement prior to revision

D.F. Hamilton*, R. Burnett, J.T. Patton, C.R. Howie, A.H.R.W. Simpson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)
5 Downloads (Pure)


Instability is the reason for revision of a primary total knee replacement (TKR) in 20% of patients. To date, the diagnosis of instability has been based on the patient's symptoms and a subjective clinical assessment. We assessed whether a measured standardised forced leg extension could be used to quantify instability.

A total of 25 patients (11 male/14 female, mean age 70 years; 49 to 85) who were to undergo a revision TKR for instability of a primary implant were assessed with a Nottingham rig pre-operatively and then at six and 26 weeks post-operatively. Output was quantified (in revolutions per minute (rpm)) by accelerating a stationary flywheel. A control group of 183 patients (71 male/112 female, mean age 69 years) who had undergone primary TKR were evaluated for comparison.

Pre-operatively, all 25 patients with instability exhibited a distinctive pattern of reduction in 'mid-push' speed. The mean reduction was 55 rpm (SD 33.2). Post-operatively, no patient exhibited this pattern and the reduction in 'mid-push' speed was 0 rpm. The change between pre- and post-operative assessment was significant (p < 0.001). No patients in the control group exhibited this pattern at any of the intervals assessed. The between-groups difference was also significant (p < 0.001). This suggests that a quantitative diagnostic test to assess the unstable primary TKR could be developed.

Original languageEnglish
Pages (from-to)1339-1343
Number of pages5
JournalBone and Joint Journal
Issue number10
Publication statusPublished - 1 Oct 2014


  • instability
  • total knee replacement
  • revision surgery

ASJC Scopus subject areas

  • Surgery
  • Orthopedics and Sports Medicine


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