Partial Knee Replacement as a Definitive Implant: 20 year in-vivo wear results.

+1Simpson, D J; 1Kendrick, B K; 1Gill, H S; 2Valstar, E; 2Kaptein, B; 3Dodd, C; 1,3Murray, D W; 1,3Price, A P

+1University of Oxford, Oxford, UK, 2Leiden University Medical Centre, Leiden, the Netherlands 3Nuffield Orthopaedic Centre, Oxford, UK

Andrew.price@ndorms.ox.ac.uk

INTRODUCTION:

Polyethylene wear in joint arthroplasty has long been held as one of

the major causes of aseptic loosening, in addition to revision of

catastrophically worn bearings. The Phase 1 Oxford partial knee

replacement (OPKR) (Figure 1) was introduced as a design against wear,

with a fully congruous articulation. In 1987 the Phase 2 implant was

introduced with new instrumentation and changes to the bearing shape to

reduce the incidence of anterior impingement; a spherical mill was

introduced for the femoral side and the anterior lip of the bearing was

reduced in height. We have previously shown that the implant has a

wear rate of 0.02 mm/year at ten years, in well functioning devices, but

that higher wear rates can be seen with impingement or if the congruous

articulation is lost. The aim of this study was to investigate whether

there is a difference in wear at 20 years between the Phase 1 and Phase 2

OPKR designs, using Roentgen Stereophotogrammetric Analysis (RSA).

METHODS:

We measured the in-vivo wear of 7 Phase 1 (5 patients, mean age

65.24 years) and 7 Phase 2 (4 patients, mean age at operation 63.43)

Oxford PKR bearings. Average time since surgery was 22.37 years and

19.46 years for the Phase 1 and Phase 2 implants respectively. Selection

criteria included patients who were mobile, with an exercise tolerance

greater than 100m as per the American Knee Society functional

questionnaire. RSA x-rays were taken with the knee in the normal

anatomical position on standing and with the knee flexed to 30°. Wear

was calculated using an RSA CAD model system, to fit silhouettes of

the prostheses to the biplanar images. The RSA software (Medis

Specials, Leiden, Netherlands) estimated the pose of both the femoral

and tibial components. For a control the bearings of ten knees receiving

PKRs were measured using this system within one week of their

operation. This provided a measure of the system accuracy as it is

assumed that no wear has occurred in the first week. The Oxford Knee

and American Knee Society Scores were obtained when patients

attended for their radiographs.

A sphere was fit to the femoral component and the bearing thickness

was determined by measuring the shortest perpendicular distance

between the two components. The linear wear for each bearing was

calculated by subtracting the measured thickness from the corrected

nominal bearing thickness.

Figure 1. Phase 1 Oxford PKR (A&C), showing different interior

geometry of the femoral component compared with phase 2 (B&D).

RESULTS:

The demographics for each of the patients are shown in Table 1.

There was no statistically significant difference in patient age but there

was a significant difference for time since surgery. There was also no

statistically significant difference for the Oxford Knee Score, the total

American Knee Society Score, or the functional section of the AKSS.

The measured wear rate was 0.072 mm/year for Phase 1 (S.D. 0.028,

range: 0.031 – 0.104 mm) and 0.028 mm/year for Phase 2 (S.D. 0.019,

range: 0.014 – 0.07 mm). This difference in wear rate for the two phases

was statistically significant (p = 0.0028).

The bearings of the ten control knees measured postoperatively showed

a mean difference of 0.02 mm between the measured thickness and the

nominal bearing thickness, with a standard deviation of 0.09 mm.

Age at

Surgery

Years

since

surgery OKS AKSS

AKSS

Function

AKSS

Total

Phase 1 mean 65.24 22.37 40.71 97.71 70.71 167.71

Phase 2 mean 63.43 19.46 39.86 89.43 70.71 160.14

p-value (t-test) 0.56 0.002 0.694 0.016 1.00 0.321

Table 1 – Patient demographics and functional scores.

Phase

Phase 2Phase 1

We

ar (

mm

/year)

0.12

0.10

0.08

0.06

0.04

0.02

0.00

n=7 n=7

Figure 2. Boxplot for each phase. Solid black line is the median, the

height of the box is the interquartile range and the whiskers are the range

of results.

DISCUSSION:

The results show that in the knees studied there was a significant

difference between wear rates in Phase 1 and 2. We propose that these

differences are explained by impingement in Phase 1, which was

reduced by design changes with the introduction of Phase 2. The

instrumentation for Phase 2 included a mill for the femoral component,

which enables more accurate placement of the femoral component and

better ligament balancing. The Phase 2 OPKR is thus designed to avoid

impingement between the femur and the bearing. This study

demonstrates that very low wear rates can be maintained with the Phase

2 implant to the end of the second decade after implantation and that low

wear designs need appropriate methods of implantation. This is of

particular importance when a device is used in younger patients, and

demonstrates that the OPKR can, in some patients, be a definitive

implant and not an interim to total knee replacement.

Paper No. 169 • 56th Annual Meeting of the Orthopaedic Research Society