The Impact of Bone Size and Proportion Resected on the Profile of the Cut Tibia

Introduction

Total Knee Replacement (TKR) is a highly effective treatment providing pain relief and improved function to patients experiencing advanced stage osteoarthritis. Tray fit or bone coverage is a critical design feature for both cemented and cementless designs affecting stability, load transfer and potential for infection. Many authors have attempted to characterise the relationship between the profile of the proximal tibia and gender and ethnicity^1–3. As a consequence, a number of manufacturers have commercialised devices designed for specific gender and racial demographics. This study was initiated to compare the effect of the fixed minimum tibial resection depth prescribed by existing surgical instruments with that of a proportionate resection based on the size of the tibia.

Method

A dataset consisting of 30 donor scans from a US cadaver tissue bank (ScienceCare, Memphis, US) was used for this study. The dataset consisted of 12 male and 18 female specimens. Due to the limited view of the diaphysis for most scans, the natural slope of the lateral compartment was used as a guide for orienting the resection. All scans were resected with a 3° posterior slope. For the first part of this study, an equal mediolateral (ML) resection of 9.5 mm, reflecting the minimum resection for the Unity TKR tibia (Corin, UK), was performed on all specimens (Figure 1). Following this, two proportionate resection depths (13.5 mm and 6.7 mm) were calculated based on the ML relationship between the smallest and largest available Unity components (59.5 mm: 84.5 mm). Two further resection depths (11.3 mm and 8.0 mm) were calculated based on a mid size (71.0 mm). Three resection depths (8.0 mm, 9.5 mm & 11.3 mm) were applied to four medium sized specimens. In addition to this two larger specimens were resected at 9.5 mm and 13.5 mm and two smaller specimens at 6.7 mm and 9.5 mm.

A grid was applied to all cut surfaces and oriented using the posterior axis. The cut surface was divided based on lines drawn at 10%, 25%, 50%, 75% and 90% of the overall ML dimension and 10%, 25%, 50%, 75% and 90% of the overall anteroposterior (AP) dimension. Measurements were taken from the medial side and recorded from the points at which lines intersected the external profile of the cut tibia (Figure 2).

Results

Results were presented as percentages relative to the AP and ML enabling the generation of 2D curve plots of the proposed profile (Figure 3).

Discussion

Results from the fixed resection (9.5 mm) data depicted a good trend (R² = 0.71–0.72) for the progression of the anterior profile as the tibia size increases. Similarly as the resection depth increased the same trend was observed. A weaker trend of R² = 0.5 was also evident for the posterior profile. This methodology was applied to the development of the Unity tibia size range to optimise bone coverage and strain distribution.