In the last years custom-fit cutting guides using magnetic resonance imaging (MRI) were introduced by orthopedic surgeons for total knee arthroplasty (TKA). One of the advantages of these shape-fitting jigs is the possibility to transfer the preoperative planning of the TKA directly to the individual patient's bone. However, one has to be aware, that the jigs are designed for single-use and have to be custom made by an external manufacturer. This increases the cost of implantation and unlinks the surgeon from this process. In addition a potentially necessary adjustment of the preoperatively planned implant size and position in a surgical situation is not possible.

The purpose of our development was to combine the advantages of custom-fit cutting guides as a 3-D-computer-assisted planning tool with the option to adjust and improve the preoperative planning and the jig in the actual surgical situation. In addition no outside jig manufacturing would occur in this concept. This leaves the surgeon in control of the entire process.

The purpose of this study was to examine the reliability of this screw-based shape – fitting system. In order to do this we assessed the inter- and intra-observer reliability of the recurrent placement of the plate on a set of bone samples with preset screws.

We developed a plate with the dimension of 66 × 76 × 10 mm, containing 443 threaded holes. A connector for further instrumentation is mounted on the proximal part of the plate,. As the plate and the screws are made of aluminum and steel, sterilization is possible.

After computer tomography (CT) scans were taken from three human femoral bones, eight to nine variably positioned screws (50.45 mm length, 2.75 mm diameter), reversibly fixed by locknuts, formed an imprint of a bone's surface. For calculating precise screw positions, a computer-based planning software was developed resulting in a three-dimensional reconstruction of the bony surfaces. The plate was integrated in the 3-D reconstruction software. With a defined distance to the distal part of the femurs, allowed the proper length and position of the screws to be calculated. These calculations were transferred to the screws on the real plate.

In the next step the plate was positioned on the bony surface and after reaching the planned position the plate's connector was rigidly fixed to the bone. The plate was removed to give place to link saw jigs to the connector.

Planning and setting of the plate and the screws were conducted on three femoral bones.

Examinations were performed by five investigators with ten repetitions on each bone with three distinct plates. Intra- and inter-observer variability was assessed by measuring the variation in plate position between the trials.

The jigs were placed in a mean frontal tilting (medial to lateral) of 0.83°. The mean axial tilting (proximal to distal) was 1.66° and the mean shift on the axis from proximal to distal 8.48 mm. The shift and the tilting were significantly bone dependent but not user dependent. Compared with previous studies the deviation from the mechanical axis were comparable with conventional TKA (2.6° and 0.4°), computer assisted TKA (1.4° and 1.9°) and Custom-fit TKA (1.2°).

We developed a preoperative planning system for TKA that allows a transfer of the planning and the calculated imprint of the bones surface on a grid-plate during surgery by the surgeons themselves. Neither external manufacturers to create a fixed device nor a navigation system is necessary. Results showed the functioning of the screw – based shape fitting technique within the accuracy mentioned above. These findings are encouraging to do further research to examine the ideal number of screws to offer a perfect fitting.

INTRODUCTION

While standard instrumentation tries to reproduce mechanical axes based on mechanical alignment guides, a new “shape matching” system derives its plan from kinematic measurements using pre-operative MRIs. The current study aimed to compare the resultant alignment in a matched pair cadaveric study between the Shape Match and a standard mechanical system.

Background: Within the scope of the EU project EUPHORIC a methodology for direct comparison of different datasets was developed and applied on a sample of implants, among them the Oxford Unicompartmental Knee Arthroplasty (Oxford Uni). The aim was to identify potential bias factors inherent in the datasets and evaluate the outcome achieved with this implant.

Materials and Methods: A structured comparison was performed of data published on the revision rate of the Oxford Unicompartmental prosthesis. Both clinical follow-up studies published in Medline-listed journals and worldwide Register data were included. The data were stratified with regard to potential influence factors like the individual research groups or the geographical origin of the papers.

Results: A major proportion of the published data, between 50 and 75%, depending on the method of calculation, comes from studies including the developing institution in Oxford. The results published by this group deviates statistically significantly from the reference datasets from Register data or independent research groups. Data from the developing hospital show mean revision rates that are 4.4 times lower than those based on worldwide Register data, and 2.74 times lower than in independent studies. As opposed to this, independent studies on average publish data that are reproducible in Register data.

Conclusion: A conventional meta-analysis of clinical studies is significantly affected through the influence of the developing institution and is therefore subject to a bias. Neither through arthroplasty Register outcome data nor by other research groups that have disclosed outcome information on the Oxford Uni can the excellent results be reproduced that were published by the inventors.

Compared to other implants for unicompartmental knee arthroplasty in worldwide arthroplasty Registers, the Oxford Uni shows good results.

For the assessment of the outcome of implants, register data are to be rated superior and, in terms of reference data for the detection of potential bias factors in the clinical literature, can provide an essential contribution for scientific meta-analyses.