Summary

In patients undergoing primary total knee arthroplasty, with no prior knee surgery, pre-operative patellar height is influenced by age, gender and overall height of the patient.

Abstract

Introduction

Alteration of patellar height is commonly encountered in total knee arthroplasty (TKA). Patients with lower pre-operative patellar height are at risk of patella baja after TKA, that may result in suboptimal functional outcomes. It is therefore prudent to determine which variables can affect patellar height and consequently increase the risk of post-operative patella baja. The purpose of this study was to assess the influence of age, gender, weight, height, body mass index and prior knee arthroscopy on pre-operative patellar height in patients undergoing primary TKA.

Methods

A retrospective consecutive case series of 317 patients who underwent primary total knee arthroplasty (TKA) between September 2012 and July 2014 was evaluated. Standard non-weight bearing lateral knee radiographs with minimum 30° flexion taken pre-operatively were assessed to measure patellar height. Pre-operative patellar height (PPH) was measured using plateau-patella angle (PPA), Blackburn-Peel ratio (BPR), Caton Deschamps ratio (CDR) and Insall Salvati ratio (ISR). 29 (9.2%) radiographs with inadequate flexion, obscured landmarks, alarming malrotation or prior history of surgery (other than plain arthroscopy) were excluded. Thus, 285 radiographs were available for evaluation and a total of 1,710 measurements were made by a single orthopaedician- knee surgery fellow. A pilot study, including 40 radiographs, was carried out prior to final measurements by two observers (fellow and fourth year resident) to reach a consensus on the technique of measurement.

Results

The average patient's age was 71 (range, 49-90) years and ninety-four (33%) were male. Average height and BMI of the cohort was 1.68 (range, 1.4-1.97) m and 30.35 (range, 19.83-49.78) kg/m2 respectively. Sixty-six (23.2%) patients had history of at least one prior arthroscopy. Mean PPH measurements were 25.40 (SD 3.650) for PPA, 0.84 (SD 0.13) for BPR, 0.87 (SD 0.14) for CDR and 1.00 (SD 0.16) for ISR. Nine (4%) radiographs had all four PPH measurements one standard deviation below the mean. This cohort had 4 (44%) males, mean age of 66 years and mean height of 1.69m. Multivariate linear regression analysis identified gender and age (p-value <0.05 for PPA and ISR) as variables significantly affecting patellar height. Male gender and younger age were found to be significantly associated with decreased PPH meaurements. Patient’s overall height had a weak negative correlation to PPH (p-value <0.05 for BPR). Weight, BMI and history of prior arthroscopy had no significant effect on any of the four PPH measurements.

Conclusion

In patients undergoing primary TKA, with no prior knee surgery, pre-operative patellar height is influenced by age and gender. Lower patellar height is significantly correlated to male gender and younger age. Overall height has a weak negative correlation, where as, weight, BMI and prior knee arthroscopy have no bearing on the pre-operative patellar height. These findings should be considered in surgical planning of primary TKA in order to avoid post-operative patella baja.