Summary

Metal-on-metal hip replacements have been associated with adverse reactions including inflammatory pseudotumours and soft tissue necrosis. We have shown that cobalt can directly activate toll-like receptor 4, an immune receptor causing pro-inflammatory interleukin-8 secretion. This may contribute to adverse reaction development.

Purpose

Although classic teaching holds that the least amount of constraint should be implanted, there is very little in the literature to substantiate this. This study attempts to quantify the influence of constraint and various indications upon functional outcome following aseptic first time revision knee arthroplasty. The null hypothesis was that the level of constraint and indication for surgery would not influence the functional outcome following revision knee replacement.

Introduction

Cemented stems have shown 90–100% survivorship when coupled with polyethylene acetabular component. This study aims to compare cemented stem behaviour in combination with large metal on metal (MOM) vs. metal on poly (MOP) bearings.

Introduction

We wished to quantify the influence of constraint and various principal indications upon functional outcome following aseptic first time revision knee arthroplasty.

During a retrospective case note analysis, a significant difference was found in prosthesis survival, between two cohorts of patients who underwent different total knee replacements. The first cohort included 70 patients who underwent Kinemax Plus total knee replacement, the second cohort included 58 patients who underwent PFC Total Knee replacement. All patients were under the care of one Consultant Orthopaedic Surgeon. Interestingly, the Kinemax Plus cohort was found to have a higher rate of revision compared to the PFC cohort. A detailed comparison was carried out between the two groups to identify any obvious cause for the disparity.

The two cohorts were found to be well matched with respect to age, sex, ASA grade, underlying pathology and operative technique. Median follow up being 6 years and 5 years for the Kinemax Plus and PFC groups respectively.

There were 11 failed prostheses in the Kinemax Plus cohort, 7 undergoing revision with the remaining 4 patients offered revision but unwilling to have surgery. Wear of the polyethylene tibial insert was the most obvious finding at revision, present in six of the seven revisions. 97% of the Kinemax Plus Prostheses were intact at 5 years but by 8 years only 87% were intact. There were no revisions performed in the PFC cohort.

Post-operative x-ray analysis was undertaken to rule out prosthesis malalignment as a cause for the increased failure rate. The coronal alignment of the prostheses (CAK) was calculated and all post operative x-rays were within the normal limits of 4-10 degrees.

Analysis of the explanted Kinemax Plus polyethylene liners was undertaken. In six cases, the polyethylene bearing surfaces displayed severe surface and subsurface delamination. This suggests massive fatigue and fatigue wear. Only one implant showed localised delamination.

These findings suggest the hypothesis of weak polyethylene particle interface strength.

Introduction and Aims: Morbid obesity (BMI> 40) has been shown to cause increased perioperative morbidity and poorer long-term implant survivorship following total knee arthroplasty (TKA). The aim of this study was to determine the impact of morbid obesity on patient-reported outcomes following TKA.

Methods: Patients undergoing primary TKA were invited to complete questionnaires preoperatively and one year after surgery. Questionnaires include the WOMAC and SF-36 health status measures, demographics, self-reported comorbid medical conditions, height and weight. At follow-up, satisfaction with results of TKA is included. Patients were categorised by their preoperative BMI categories as ideal weight (20–25), overweight (> 25–30), obese (> 30–40) or morbidly obese (> 40). We used multivariate analysis to adjust for known significant correlates of WOMAC and SF-36, namely age, gender and comorbid medical conditions. Adjusted mean scores for each assessment were compared by BMI category.

Results: A total of 769 patients were included in the study which included 27 morbidly obese, 280 obese, 314 overweight and 148 ideal weight patients. Morbidly obese patients when compared with non-obese patients had significantly worse preoperative WOMAC pain and function and a trend for worse SF36 scores (Vitality significantly worse, p=0.04). There was no significant difference between the BMI categories at one year for WOMAC or SF-36 scores (trend for the Physical Functioning score to be lower, p=0.052). Morbidly obese patients were all satisfied with pain relief after surgery but were less satisfied with functional results when compared to the non-obese groups.

Discussion: This study shows that morbidly obese patients report significantly worse health status prior to TKA but their post-operative improvement is greater than the non-obese population. Although morbid obesity should not be an absolute contraindication to TKA, patients should be carefully selected balancing the risk of perioperative complications and earlier failure against the demonstrated marked improvement in quality of life.

Background: Resurfacing cups may produce significant clinical problems when placed at the extremes of version, including increased production of metal debris and psoas tendonitis.

Methods: We obtained the femoral and acetabular components of two unused ASR (Articular Surface Replacement) and BHR (Birmingham Hip Resurfacing) devices. The components were placed in moulds at varying degrees of inclination and anteversion in a grid to simulate pelvic landmarks and AP radiographic images were captured. The true radiological anteversion was determined by rotating the x-ray beam so that it was perpendicular to the acetabular axis and measuring the angle subtended by the cup rim and the vertical axis.

Five blinded orthopaedic registrars then used EBRA (Einzel-Bild-Roentgen-Analysis, University of Inns-bruck, Austria) software to determine the radiological anteversion from the AP films. Twenty-five ASR and twenty-five BHR images were analysed.

At the same time each observer was asked to grade the cups as “1” (< 10°) “2” (10–20°) “3” (20–30°) or “4” (> 30°) depending on the appearances of the cup vertices.

Results: Mean error for each observer was −0.7 (minimum) to 1.6° (maximum). The range of standard deviations of error for each observer was +/− 2.2 (minimum) to 3.5° (maximum). Retroverted cups were not identified in the majority of cases.

Cups graded as “1” or “4” showed high sensitivity and specificity for the true grade as determined on the lateral radiographs.

Conclusions: EBRA software can be used to calculate the anteversion of resurfacing cups to a clinically acceptable degree. The clinician must be aware of the limitations of the software most notably the difficulty in identifying a retroverted cup and errors arising from poor quality radiographs in terms of pelvic rotation. The presented clinical grading system can be used as a rapid assessment tool to identify cups at the extremes of anteversion.

The Kinemax Plus knee replacement has a reported 10 year survival of around 96%. However we found the survival rate of this implant in our cohort to be 75% at 9 years. No abnormalities were found for clinical and radiological parameters. At reoperation the most striking feature was that of significant ultra-high molecular weight polyethylene (UHMWPE) failure. Oxidative and structural analysis of the polyethylene components was therefore undertaken.

Ten Kinemax Plus tibial inserts were analysed; one was a shelf-aged unused implant, the others were explants. An FTIR analysis of the data showed that oxidation is present in all samples. The degree of oxidation however varied with depth and location. Except for a sharp oxidative peak approximately half way into the sample, the shelf aged samples had a fairly constant level of oxidation. The retrieved implants had an overall higher level of oxidation in both bearing and non-bearing regions. The latter had less of a variation in oxidation which implies that in vivo loading exaggerates the degree of oxidation. In the non-articulating regions oxidation of the explants was found to peak often at the region of about 40% from the bottom surface in all retrieved samples. By contrast, most articulating region had two oxidative peaks; one occurring at approximately 1–1.5mm from the surface, which is consistent with findings on subsurface oxidation, and another occurring about 2–3mm from the bottom surface.

SEM imaging provided evidence for the presence of fusion defects by indicating grain boundaries through-out the explants. This indicates a compromised material which is more susceptible to damage. Fatigue loading of the implant has also been seen to produce a subsurface stress maximum at approximately 1 to 2mm below the articulating surface. It is thought that maximum contact stresses within this region cause Type 1 and Type 2 defects to open or become more pronounced. This in turn will increase the local concentration of oxygenating material as it will be present in these defects and voids where surface areas are greater for oxidative reaction. We therefore hypothesise that these fusion defects are the cause for the early failure of the Kinemax implants.

We aim to explain the significant difference in survivor-ship found between two cohorts of patients who underwent different total knee replacements. The first cohort included 70 patients who underwent Kinemax Plus total knee replacement, the second cohort included 58 patients who underwent PFC Total Knee replacement. All patients were under the care of one Consultant Orthopaedic Surgeon.

Interestingly, the Kinemax Plus cohort was found to have a higher rate of revision as compared to the PFC cohort. A detailed comparison was then carried out between the two groups to identify any obvious cause for the disparity.

The two cohorts were found to be well matched with respect to age, sex, ASA grade, underlying pathology and operative technique. Median follow up being 6 years and 5 years for the Kinemax and PFC groups respectively. There were 11 failed prostheses in the kinemax cohort, 7 undergoing revision with the remaining 4 patients offered revision but unwilling have surgery. Wear of the polyethylene tibial insert was the most obvious finding at revision, present in six out of the 7 revisions. 97% of the Kinemax Plus Prostheses were intact at 5 years but by 8 years only 87% were intact.

There were no revisions performed in the PFC cohort.

Post operative x-ray analysis was undertaken to rule out prosthesis misalignment as a cause for the increased failure rate. The coronal alignment of the prostheses (CAK) was calculated and all post operative x-rays were within the normal limits of 4–10 degrees.

Analysis of the explanted Kinemax Plus polyethylene liners was undertaken. In six cases, the polyethylene bearing surfaces displayed severe surface and subsurface delamination at both medial and lateral sides. This suggests massive fatigue and fatigue wear. Only one inplant showed localised delamination. The surface characterisation suggests the hypothesis of weak UHMWPE particle interface strength.

We describe a cohort of patients with a high rate of mid-term failure following Kinemax Plus total knee replacement inserted between 1998 and 2001. This implant has been recorded as having a survival rate of 96% at ten years. However, in our series the survival rate was 75% at nine years. This was also significantly lower than that of subsequent consecutive series of PFC Sigma knee replacements performed by the same surgeon. No differences were found in the clinical and radiological parameters between the two groups. At revision the most striking finding was polyethylene wear. An independent analysis of the polyethylene components was therefore undertaken. Scanning electron microscopy revealed type 2 fusion defects in the ultra-high molecular weight polyethylene (UHMWPE), which indicated incomplete boundary fusion. Other abnormalities consistent with weak UHMWPE particle interface strength were present in both the explanted inserts and in unused inserts from the same period.

We consider that these type 2 fusion defects are the cause of the early failure of the Kinemax implants. This may represent a manufacturing defect resulting in a form of programmed polyethylene failure.

We report the results of a prospective randomised controlled clinical trial assessing the radiosterophotogrametric analysis (RSA), clinical and radiological performance of a metal backed and an all-polyethylene tibial cruciate retaining, condylar design, PFC-TKA up to twenty four months.

65 patients were recruited, of which 41 patients were randomised. There were 20 metal backed and 21 all-polyethylene. None were lost to follow-up. There were no significant inter-group demographic differences. We found a significant increase in SF-12 and Oxford knee scores after surgery in both groups. No significant difference was found between the groups in the RSA, SF-12, Oxford Knee score, radiological alignment and range of movement at 6, 12 or 24 months. At 2 years one metal backed implant showed translational migration > 1mm. No all polyethylene implant migrated > 1mm. Further analysis identified possible progressive subsidence of the metal backed implants compared to all-polyethylene implants, although the magnitude of this difference was very small.

We conclude that in the uncomplicated primary total knee arthroplasty, all polyethylene PFC-_ tibial prostheses had equivalent performance to the metal backed counterpart, using RSA as the primary assessment instrument at 24 months. We found no differences between the two designs as assessed by the secondary instruments: SF-12, Oxford knee score, alignment and range of movement at 24 months. Should half of all primary total knee replacements performed in the UK receive an all-polyethylene tibial implant, the estimated annual cost saving would be 21 million pounds per annum.

Introduction: Patients following resurfacing frequently remark about the natural feel of the resurfaced hip joint in contrast to those with total hip arthroplasty. Possible reasons for this include the larger femoral head size, conservation of bone and superior biomechanics of the implant with more accurate restoration of femoral offset, leg length and femoral anteversion.

Our aim was to assess femoral offset and leg length following hip resurfacing and hybrid THR (uncemented acetabulum) performed by the same surgeon.

Methods: A consecutive group of patients were identified (35 resurfacing and 25 hybrid). AP pelvis radiographs were evalulated, films with evidence of malrotation or inadequate imaging of the femur were excluded, leaving 21 resurfacing and 15 hybrid. Comparison was made between the pre-op and post-op films together with the contralateral hip on the same film. Patients with hip dysplasia or significant pathology in the contralateral hip were excluded. Magnification of the films was measured by comparison of the templated diameter of the implanted femoral head and the acutal diameter of the implant. To allow comparison between pre-op films a measurement was taken between the obturaror foraminae.

All films were analysed by the same investigator using the technique described by Jolles et al (J Arthroplasty 2002). A horizontal line was drawn between the base of the teardrop on both sides, and perpendicular lines drawn from the back of the teardrops. The anatomical femoral axis was drawn and femoral offset measured from this. The centre of rotation of the femoral head was determined by templating and the acetabular offset obtained.

Distance from tip of the greater trochanter to the centre of the femoral head in the axis of the femur was determined on pre and post-op films, as this shows little variation with rotation of the femur. Leg length was measured from the horizontal line to the tip of the greater trochanter together with the angle between the femoral axis and the horizontal to correct for abduction of the hip.

Results: Mean total femoral offset compared to the contralateral side was −1.3mm (SD 5.3) and −3.2mm (SD 6.5) for the resurfacing and hybrid groups respectively. No significant difference was detected in leg length or other measurements.

Discussion and Conclusion: No significant differences were demonstrated between femoral offset or leg length in the resurfacing and hybrid arthroplasty groups. This study does not support the hypothesis that resurfacing produces more accurate restoration of hip biomechanics than hybrid total hip arthroplasty.

Aims: To review the early results of Birmingham hip resurfacing (BHR). First 116 hips (98 patients). Materials: A cohort of consecutive patients prospectively underwent BHR in a single arthroplasty Surgeonñs practice in a University Hospital out side Birmingham. Inclusion criteria þt and active patients expected to out live conventional THR. 98 patients mean age 50 years (range 19–67). Pathology OA 85%, Perthes 7%, DDH 4.7% & SUFE 2%. Mean follow-up 30 months (range 12–45). Scoring systems used were Harris hip score (HHS), WOMAC & SF 36. Results: All patients under review, no radiological loosening.

HHS:Mean pre-arthroplasty score 47 (range 10–73), 1 yr score 99 (n=38), 2yr 97.3 (n=30) at 3yr 100 (n=26) & 4th yr 100 (n=4), this shows a statistically signiþcant improvement (P=0.001).

WOMAC: Pain: pre-op score 18.8, at 1 year 5.6 & 2nd year 5.7. Stiffness: pre-op 8.5, 1 year 2.7 & 2nd year 2.7. Physical: 49.3, 1 year 23.4 & 2nd year 22.6.

This shows a statistically signiþcant improvement in score (p=0.025) SF-36 Analysed against an age/sex match control group of normal values using a 2 tailed Ôtñ Test, seven of the eight domains showed no statistical signiþcant difference. Complications: One fracture following a fall, 2 AVN & early fractures (patients had deep cysts in proximal head now such patients are not offered BHR), one dislocation following RTA Conclusions: Birmingham hip resurfacing can provide excellent level of activity and patient satisfaction. Careful patient selection is needed. A long-term study is needed to evaluate the long-term beneþt and survivorship

To review the early functional results of Birmingham hip resurfacing. First 116 hips (98 patients), a cohort of consecutive patients prospectively underwent BHR in a single arthroplasty Surgeon’s practice in a University Hospital outside Birmingham. Inclusion criteria fit and active patients. 98 patients mean age 50 years (range 19–67). Pathology OA 85%, Perthes 7%, DDH 4.7% & SUFE 2%. Mean follow-up 30 months (range 12–45). Scoring systems used were Harris hip score (HHS), WOMAC & SF36.

HHS: mean pre-arthroplasty 47 (range 10–73), one year 99 (n=57), at 2 years 97.3 (n=26) and at third year 100 (n=3), statistically significant improvement (P=0.001).

WOMAC: pain: pre-op score 18.8, at 1 year 5.6 and second year 5.7. Stiffness: pre-op 8.5, 1 year 2.7 & 2^nd year 2.7. Physical: 49.3, 1 year 23.4 & 2^nd year 22.6.

This shows a statistically significant improvement in pain score (p=0.025) and physical function score (p=0.025).

SF-36 one-year post op, when analysed against an age/sex match control group normal values using a 2 tailed ‘t’ Test, seven of the eight domains showed no statistical significance. Only the Social Functioning domain showed a statistically significant result (p=0.011).

One fracture following a fall (patient had deep cysts in proximal head now such patients are not offered BHR), one dislocation following RTA and one AVN in a 50 years old post menopausal lady.

Birmingham hip resurfacing can provide excellent level of activity and patient satisfaction. A long-term study is needed to evaluate the long-term benefit and survivorship.

Dual Energy X-ray absorption (DEXA) has been used to measure bone mineral density (BMD) around total hip prostheses. With the recent increase in the use of metal on metal hip resurfacing, such as the Birmingham Hip Resurfacing (BHR), there has been renewed concern over per prosthetic femoral neck fracture and implant loosening. DEXA quantitatively measures bone mineral density and therefore could predict impending loosening and fracture. To the best of our knowledge, there are no recorded studies assessing BMD around metal-on-metal hip resurfacings such as the BHR. Our intention was to produce a reliable method of measuring bone density around a metal-on-metal hip resurfacing, such as the BHR, prior to a prospective study.

We performed DEXA scans on five patients (7 BHR’s), who had undergone resurfacing with the BHR within the last two years, using the Hologic QDR 45000A scanner. Each BHR was scanned twice on the same day with complete patient repositioning between scans. We analysed the data with the Hologic prosthetic hip (v 8.26a: 3) scan analysis software (operating software 9.80D) by identifying a variable number of same-sized regions of interest (ROI) within the femoral neck. These ROI’s were derived from an inter-trochanteric line and the axis of the BHR stem in the femoral neck. Each of the 14 scans was analysed twice, by three of the authors independently; with at least one week between repeat analysis by the same observer. Statistical analysis was carried out by the local University Department of Statistics.

The variation within the same ROI in a given BHR was 0.00353, whereas the variation between all ROI’s was 1.155. The intraclass-correlation was 0.997 (i.e. the correlation between any two assessments of one ROI) with an overall coefficient of variation of 5%. The variation between the two scans for each BHR and between the three assessors was not significant (p=0.87 and p=0.42 respectively). The mean BMD of the individual ROI’s, between the two assessments of the same scans by the same assessor (i.e. intra-observer variation) was lower on the second assessment by 0.0214gcm⁻² (SD=0.0025) representing 0.5% mean density for all ROI’s. This difference was statistically significant (p< 0.001).

This method demonstrates excellent reproducibility of the method. Inter-scan and inter-observer variation was so negligible that a tiny intra-observer variation of 0.5% (of mean bone density) became statistically significant (p< 0.001), despite it making no difference to the overall intraclass-correlation. Statistical advice suggested that this very small difference in mean density (intra-observer) only reached significance due to the highly sensitive measurements and excellent reproducibility.

We have designed and demonstrated a safe, non-invasive and highly reproducible method for scanning BHR implants in vivo using DEXA. We will now use this method to prospectively study our BHR population to detect impending loosening or fracture.