Introduction

This study assessed outcomes of total knee joint replacements (TKJR) in patients who had undergone previous periarticular osteotomy compared with unicompartmental knee replacement (UKR). Establishing a difference in the results of total knee joint replacements following these operations may be an important consideration in the decision-making and patient counselling around osteotomy versus UKR for the management of single-compartment osteoarthritis.

Digital radiographs have taken over from conventional radiographs in most of the hospitals in New Zealand. This has created a challenge with respect to templating and pre-operative planning of total hip replacement surgeries. Digital templating has not, until recently, been available in our hospitals. Recently, a digital templating system (Cedara) has become available and has been installed and used at Middlemore Hospital and at Manukau Surgical Centre. This system allows computerised templating of digital radiographs. The aim of this study was to assess the accuracy of digital templating and to compare this method to the “compromised” conventional templating that has been performed at Middlemore hospital for the last 10 years.

In order to correct for magnification a fiducial stand has been created. This is a plastic stand and a pole with a movable 36mm metal ball. The ball is placed at the level of the greater trochanter and the stand is then placed between the patients legs. A standard templating “AP pelvis for hips” radiograph is then taken with the limbs internally rotated, such that the patellae are facing anteriorly. A traditional hard film was then created from this image for traditional acetate templating. Digital templating, with the Cedara system, was performed on the digital images after calibrating the image size using the fiducial image of known size. The results of the two methods were tabulated. The operation record was read and the component size and type was tabulated. The postoperative radiographs were assessed and the component positioning was evaluated and compared with the conventional and digital templates. A critical assessment of component size, with respect to under sizing and over sizing, was also performed on the postoperative radiographs and this was correlated with the digital and conventional templates.

This templating, and the evaluation of the postoperative radiographs, was performed by the authors of this paper (a consultant surgeon and a registrar). The accuracy of the two templating methods was assessed by comparison with the post operative radiographs and also with the aid of the knowledge of the actual components which were used at the time of surgery.

The templating images and radiographs of 100 patients were evaluated in the above stated manner and the results were analysed. The results from this analysis will be presented.

Correct sizing of knee arthroplasty implants avoids problems such as stiffness from too large an implant, or periprosthetic fractures from undersizing. Currently most implants are based on a generic unisex population. Femoral component sizing is therefore based solely on the AP measurement after the distal femoral cut. In order to investigate the differences between the New Zealand population and other populations with reported anthropometrics we studied the anthropometrics of the male and female distal femur.

The distal femur of 26 cadaveric knees was resected using standard cutting guides. Using a sizing guide the AP dimension was measured from the posterior condyle to the anterior cortex just proximal to the trochlea (posterior referencing). The ML dimension was measured at the cut surface in the coronal plane of the epicondylar axis.

Overall AP measurement had a mean(standard deviation) of 62(±6.7) mm, the ML measurements had a mean (sd.) of 72(±6.6)mm yielding an ML/AP(100) ratio of 117(±11). The male AP mean was 67(±4.5) mm and female AP 57 (±4.4)mm. The male ML was 77 (±4.7)mm and female ML 68 (±4.5)mm. The ML/AP ratio for male was 111(±12) and female was 120 (±10).

This pilot study has shown differences between genders in the NZ population even with this small sample size. As this data is important for designers of total knee implants, planning is currently underway to perform measurements intraoperatively from approximately 400 patients undergoing total knee replacement.