Aims

This aim of this study was to assess the feasibility of designing and introducing generic 3D-printed instrumentation for routine use in total knee arthroplasty.

Introduction

In major orthopaedic departments, typically several total knee systems are used. Each system requires several sets of instruments, each set with many trays of complicated and expensive parts. The logistics and costs of maintainance are considerable. Our overall goal is to investigate the feasibility of autoclavable single-use 3D printed instruments made from a polymeric material, used for any type of total knee design. The procedure will be standardized and adjustments easy to implement. Each set will be packaged individually, and used for a single case. There are many aspects to this study; in this part, the aims are to identify suitable materials for autoclavability and strength, and then to compare the accuracy of a novel design of 3D printed tibial cutting guide with a current metallic guide.

INTRODUCTION

Soft tissue balancing in knee arthroplasty remains an art. To make it a science reliable quantification and reference values for soft tissue tension and contact loads are necessary. This study intends to prove the concept of a compartmental load safe target zone as a clinical tool for balancing total knee arthroplasties by studying the relationship between post- balancing compartmental load distribution and patient satisfaction at 6 months.

We present the medium to long term clinical results of 112 Uncemented custom Computer Assisted Design Computer Assisted Manufactured (CAD-CAM) total hip arthroplasties performed between 1992 and 1998 in 111 patients. Fifty three males and 58 females were included. Mean age was 46.2 years (range 24.6yrs - 62.2 yrs). Average duration of follow up was 156 months (120 – 204 months). The mean Harris Hip Score (HHS) improved from 42.4 to 90.3, mean Oxford Hip Score (OHS) improved from 43.1 to 18.2 and the mean WOMAC hip score improved from 57.0 to 11.9. There was 1 revision due to failure of the acetabular components but there were no failures of the femoral components. There were no revisions for aseptic loosening. The worst case survival in this cohort of custom femoral components at an average 13 year follow up (range 10-17 years) was 98.2% (95% Confidence interval 95% to 99%). Survival of the femoral component alone was 100%.

These results are comparable with the best medium to long term results for femoral components used in primary total hip arthroplasty (THA) with any means of fixation.

The objective was to develop a simple, rapid, and low-cost method for evaluating proposed new Total Knee (TKA) designs, and then to use the method to evaluate three different TKA models with different kinematic characteristics. In a previous study, we reported on the use of an Up-and-Down Crouching Machine, where the neutral path of motion for knee specimens were measured, and then TKR models were implanted and the tests repeated. These experiments showed that standard CR and PS designs behaved more like an ACL deficient knee, whereas Guided Motion knees produced motion similar to that of the intact specimens. However the method was time consuming, technically demanding, and expensive, and hence is suitable for designs which had already passed through a screening method. The latter was the subject of this present study, called the Desktop TKR Test Machine.

The principle of the testing protocol on the machine, called Holistic Testing, was that a spectrum of compressive, shear and torque forces were applied to a knee, to represent a complete spectrum of daily and sporting activities. The resulting femoraltibial positions were measured, both the Neutral Path of Motion and the Laxity about the neutral path. The motions were displayed as both the motion of the transverse femoral axis on the tibial surface, and by the centers of the lateral and medial contact patches.

Eight knee specimens were tested first, to act as a reference target for evaluating TKR models. Knee models were designed in the computer and made in a hard low-friction plastic using SLA and stereolithography.

A typical Posterior-Stabilized (PS) TKA did not display the normal external femoral rotation with flexion, and also showed abnormal anterior sliding on the medial side prior to cam-post engagement. Guided Motion designs included a Medial Pivot type, and a Medial Pivot with a cam-post. Both of these had a dished medial side and a shallow lateral side, to more accurately reproduce anatomic motion characteristics. The guidedmotion design with the cam-post produced a neutral path and laxity more similar to that of normal.

It was concluded that the test method satisfied the objective in terms of being a useful test method for rapid evaluation of new proposed TKR designs. The method was able to identify designs which showed motion and stability characteristics closer to the normal anatomic knee.

We prospectively evaluated the long-term outcome of 158 consecutive patients who underwent revision total hip replacement using uncemented computer-assisted design-computer-assisted manufacture femoral components. There were 97 men and 61 women. Their mean age was 63.1 years (34.6 to 85.9). The mean follow-up was 10.8 years (10 to 12).

The mean Oxford, Harris and Western Ontario and McMaster hip scores improved from 41.1, 44.2 and 52.4 pre-operatively to 18.2, 89.3 and 12.3, respectively (p < 0.0001, for each). Six patients required further surgery. The overall survival of the femoral component was 97% (95% confidence interval 94.5 to 99.7). These results are comparable to those of previously published reports for revision total hip replacement using either cemented or uncemented components.

We present the 10- to 17-year results of 112 computer-assisted design computer-assisted manufacture femoral components. The total hip replacements were performed between 1992 and 1998 in 111 patients, comprising 53 men and 58 women. Their mean age was 46.2 years (24.6 to 62.2) with a mean follow-up of 13 years (10 to 17). The mean Harris Hip Score improved from 42.4 (7 to 99) to 90.3 (38 to 100), the mean Oxford Hip Score from 43.1 (12 to 59) to 18.2 (12 to 51) and the mean Western Ontario MacMasters University Osteoarthritis Index score from 57.0 (7 to 96) to 11.9 (0 to 85). There was one revision due to failure of the acetabular component but no failures of the femoral component. There were no revisions for aseptic loosening. The worst-case survival in this cohort of custom femoral components at 13.2 years follow-up was 98.2% (95% confidence interval 95 to 99). Overall survival of this series of total hip replacements was 97.3% (95% confidence interval 95 to 99).

These results are comparable with the best medium- to long-term results for femoral components used in primary total hip replacement with any means of fixation.

Most navigation systems for TKR help in the alignment of bulky cutting jigs. We hypothesized that TKR bone cutting could be done free hand without cutting jigs, by navigating a bone saw directly. This would allow smaller incisions, faster recovery time and simpler procedures. The goal of this study was to evaluate the results of free-hand cutting by using in-house developed CAOS software against cuts with traditional jigs.

Experiments were carried out on the five planar cuts of the TKR distal femur, using first the conventional cutting jig and then freehand. The Freehand cutting system navigated and displayed 3D realistic models of the saw, the bone and the planes along which the blade should be orientated. Two experienced arthroplasty surgeons and one engineer performed the experiments on 18 identical synthetic femurs. Each performed one using jigs and five freehand. The experiments were timed and > 50 direct measurements were made for each (cut) bone with a computer digitizer, digital caliper and protractor to assess their quality.

Surgeon’s comments, qualitative and quantitative assessments of the cuts proved the concept’s feasibility and its encouraging potential. The engineer’s time improvement with freehand navigation has implications for easier TKR for trainee surgeons.

Between June 1991 and January 1995, 42 hydroxyapatite-coated CAD-CAM femoral components were inserted in 25 patients with inflammatory polyarthropathy, 21 of whom had juvenile idiopathic arthritis. Their mean age was 21 years (11 to 35). All the patients were reviewed clinically and radiologically at one, three and five years. At the final review at a mean of 11.2 years (8 to 13) 37 hips in 23 patients were available for assessment.

A total of four femoral components (9.5%) had failed, of which two were radiologically loose and two were revised. The four failed components were in patients aged 16 years or less at the time of surgery. Hydroxyapatite-coated customised femoral components give excellent medium- to long-term results in skeletally-mature young adults with inflammatory polyarthropathy. Patients aged less than 16 years at the time of surgery have a risk of 28.5% of failure of the femoral component at approximately ten years.

We have performed endoprosthetic replacement after resection of tumours of the proximal tibia on 151 patients over a period of 20 years. During this period limb-salvage surgery was achieved in 88% of patients with tumours of the proximal tibia. Both the implant and the operative technique have been gradually modified in order to reduce complications. An initial rate of infection of 36% has been reduced to 12% by the use of a flap of the medial gastrocnemius, to which the divided patellar tendon is attached. Loosening and breakage of the implant have been further causes of failure. We found that the probability of further surgical procedures being required was 70% at ten years and the risk of amputation, 25%. The development of a new rotating hinge endoprosthesis may lower the incidence of mechanical problems.

Limb salvage for tumours of the proximal tibia is fraught with complications, but the good functional outcome in successful cases justifies its continued use.

We report our results in 24 children with malignant primary bone tumours of the distal femur treated with a Stanmore extendible endoprosthesis (SEER). This consists of a femoral component that can be lengthened, a constrained knee and an uncemented sliding tibial component which crosses the proximal tibial physeal plate perpendicularly.

The average age of the patients at diagnosis was ten years and the mean follow-up was 4.7 years (2.5 to 7.9). The mean growth of the affected tibia was 76% (18 to 136) and of the fibula 83% (15 to 750) of the growth of the unaffected limb.

Measurement of growth arrest lines showed that the mean growth of the proximal tibial physis on the affected side was 69% (43 to 100) of that of the normal side. The great variability in the growth of the physis cannot yet be explained.

We have made a retrospective study of 1001 custom-made prostheses used as replacements after surgery for bone tumours. There were 493 distal femoral, 263 proximal femoral and 245 proximal tibial prostheses. Aseptic loosening was shown to be the principal mode of failure of the implants, and 71 patients had revision for aseptic loosening of a cemented intramedullary stem.

The probability of a patient surviving aseptic loosening for 120 months was 93.8% for a proximal femoral replacement, 67.4% for a distal femoral prosthesis and 58% for a proximal tibial implant. In patients with distal femoral replacements the age of the patient at the time of operation and the percentage of bone resected were related to the risk of aseptic loosening. Young patients with distal femoral prostheses in whom a high percentage of the femur had been replaced had the poorest prognosis for survival without aseptic loosening. The percentage of bone removed had a significant effect in the proximal tibial replacement group, but the age of the patient did not. By contrast, neither the age nor the percentage of bone removed was a factor after proximal femoral replacement.

The significance of these findings is discussed in relation to mechanical factors.