To compare tourniquet times of standard and computer assisted total knee arthroplasty in patients with BMI more than 30 To evaluate the change in this variable as a surgeon gained experience over a three year period.
Group1 had average tourniquet times of 95.69 and 111.67 minutes in the standard and computer assisted groups respectively (p 0.01). Group 2 tourniquet times were 80.75 and 92.33 minutes (p 0.05). Group 3 tourniquet times were 84.5 and 87.5 minutes; these were not significantly different.
There has been renewed interest in metal-on-metal bearings as hip resurfacing components for treatment in young, active patients. This study examines the effects of fixation (cemented or uncemented heads) and bone-implant interface conditions (stem-bone and head-bone) on the biomechanics of the Birmingham hip resurfacing (BHR) arthroplasty, using high resolution, 3-d computational models of the bilateral pelvis from a 45-year-old donor. Femoral bone stress and strain in the natural and BHR hips were compared. Bone remodelling stimuli were also determined for the BHR hips using changes in strain energy. Proximal femoral bone stress and strain were non-physiological when the BHR femoral component was fixed to bone. The reduction of strain energy within the femoral head was of sufficient magnitude to invoke early bone resorption. Less reduction of stress was demonstrated when the BHR femoral component was completely debonded from bone. Bone apposition around the distal stem was predicted based on the stress and strain transfer through the stem. Femoral stress or strain patterns were not affected by the type of fixation medium used (cemented vs. Uncemented). Analysis of proximal stress and strain shielding in the BHR arthroplasty provides a plausible mechanism for overall structural weakening due to loss of bony support. It is postulated that the proximal bone resorption and distal bone formation may progress to neck thinning as increasing stress and strain transfer occurs through the stem. This may be further exacerbated by additional proximal bone loss through avascular necrosis. Medium term retrieval specimens have shown bone remodelling that is consistent with our results. It is unclear if the clinical consequences of neck thinning will become more evident in longer-term follow-ups of the BHR.
One potential limitation with uncemented, hemispherical metal-backed acetabular components is stress shielding of bony structures due to the mismatch in elastic modulus between the metal backing and the peri-prosthetic bone. A proposed substitute is a horseshoe-shaped acetabular component, which replicates the bony anatomy. One such device, the Cambridge cup, has shown successful clinical and radiological outcomes at five years follow-up (Brooks 2004, Field 2005). We conducted a study of the Cambridge cup from a biomechanical perspective, using validated, high-resolution computational models of the bilateral hip. Peri-prosthetic stress and strain fields associated with the Cambridge cup were compared to those for the natural hip and a reconstructed hip with a conventional metal-backed hemispherical cup during peak gait loading. We found that the hemispherical cup caused an unphysiologic distribution of bone stresses in the superior roof and unphysiologic strain transfer around the acetabular fossa. These stress distributions are consistent with bone remodelling. In contrast, the peri-acetabular stresses and strains produced by the Cambridge cup differed from the natural hip but were more physiologic than the conventional hemispherical design. With the Cambridge cup, stresses in the superior acetabular roof, directly underneath the central bearing region, were greater than with the conventional design. Despite the thin bearing, the peak liner stresses in the Cambridge cup (max. tensile stress: 1.2 MPa; yield stress: 4.5 MPa) were much lower than the reported material strengths. Fossa loading by the hemispherical cup has been suggested as a possible mechanism for decreased implant stability (Widmer 2002). Conversely, the Cambridge cup produced semi-lunar peri-prosthetic stress fields, consistent with contact regions measured in natural hips (Widmer 2002). These analyses provide a better understanding of the biomechanics of the reconstructed acetabulum and suggest that a change in component geometry may promote long-term fixation in the pelvis.
Operation data is now entered onto the database by the surgeon or co-ordinator at the time of surgery. Thereafter, the database automatically produces annual Oxford Hip Questionnaires, EQ-5D questionnaires and invite letters to patients for clinical review at stipulated time-points. Questionnaires are returned by patients and scanned. This data is then electronically imported to the database without transcription error. Patients attend special Outcome clinics, staffed by Research Fellows and SpR’s, who examine the relevant hip and review their radiographs. The findings are recorded and the paper forms scanned and imported into the database. Non-responders are identified from the database and are chased up via telephone by the coordinator. Data is extracted from the database with queries and presented using database reports.
We report a three year Medical Devices Agency and Local Ethical Committee approved prospective study for a new tri-tapered polished cannulated cemented femoral component. Our stem was implanted in 53 primary total hip replacements. Eleven male patients (11 hips) and 39 female patients (39 hips). The mean age at surgery was 73 (range 65 to 84). The mean weight was 71.76 kg (range 49.3 kg to 94.6 kg) with a mean BMI of 28 (range 20.20 to 40.26). All patients had a pre operative diagnosis of osteoarthritis. All the hips were implanted via the anterolateral approach. Twenty-six (51%) hips were implanted by a single consultant and 24 (49%) were implanted by six different registrars. Pre-operative and sequential post-operative clinical and radiological evaluations were undertaken. The mean pre-operative Oxford hip score was 47 points.which declined 19 points at three years. Radiological analysis, using the Johnston criteria, did not reveal any untoward features. Prosthetic stem migration was measured using a technique developed in our unit and validated as accurate to 0.61 mm; as previously reported. Stem migration measured averaged 1.38 mm (n=52; sd ±1.38) 6 months post implantation. This progressed to 1.71 mm (n=50; SD=1.18) at one year; 1.61mm (n=48; sd ±1.17) at 2 years. and 1.55 mm (n=28; SD 1.13). At 3 years average stem migration for hips implanted by the registrar group and the consultant group was not sig-nificantly different (p=0.2048) and the migration curve, against time was similar for both groups. Our study has demonstrated initial component migration, comparable to that of other polished tapered cemented stem designs. The improvement in Oxford hip score parallels other reported series and no adverse radiological signs have been observed. Long-term surveillance of our cohort will provide further data to compare the new design with substantial equivalents. More sophisticated studies, such as RSA analysis would provide further data on early femoral component migration.
Radiographic results showed no evidence of loosening of HA coated cups, in contrast to non HA coated cups which migrated significantly in 80% of cases. Four patients with loose non HA coated cups underwent revision surgery.
The novel horseshoe shaped cup was designed by the senior authors to minimise the resection of healthy bone in total hip arthroplasty. It replaces the cartilage and underlying sub-chondral bone of the acetabulum socket with a cup that is designed to flex in harmony with the surrounding bony structure. Fifty female patients with a displaced, subcapital, femoral neck fractures were chosen for the study. In half of the group of patients, the composite support shell was coated with HA, with the other half remaining uncoated. Clinical and radiological assessments were undertaken regularly for five years. To date 20 patients have died and 13 have withdrawn from the study due to poor medical health unrelated to the study. Charnley modified Merle d’Aubigne score at five years was as good as the preoperative score with 80% of patients having full range of movement, no pain and walking unaided. Radiographic results showed no evidence of loosening of HA coated cups, in contrast to non HA coated cups which migrated significantly in 80% of cases. Four patients with loose non HA coated cups underwent revision surgery. This trial has demonstrated success at 5 years with the HA coated Cambridge Acetabular Cup. Cups from which HA coating has been removed have migrated significantly in 80% of cases. There is an advantage of the HA fixation which will be taken into account before wider clinical usage is advocated.
Hip resurfacing is widely recognised as a bone conserving procedure with respect to proximal femoral resection. However, it has been argued that this is not the case for the acetabulum due to the thickness of the acetabular component and the large diameter bearing surfaces. We have investigated whether the Birmingham Hip is a bone conserving procedure with respect to the acetabular bone stock. Data was obtained from 257 consecutive Midland Medical Technology (MMT) surface replacements and 458 primary hybrid total hip replacements implanted under our care. The surface replacement group comprised 185 males (185 hips) and 72 females (72 hips) with a mean age at surgery of 55 years. The hybrid primary total hip replacement group comprised 207 males (207) and 251 females (251 hips). The mean age at surgery was 65 years old. In the surface replacement group the mean uncemented acetabular size implanted was 54.88 mm (females = 51.9 mm; males = 57.8 mm). In the hybrid primary total hip replacement group the mean uncemented acetabular size of 55.04 mm (females =52.9 mm; males = 57.2 mm). Statistical analysis was undertaken to compare the uncemented acetabular sizes in the surface replacement group with the uncemented acetabular sizes implanted in the primary hybrid total hip replacement group. We report no significant difference in the size of acetabular component used for the two groups (p = 0.4629; 95% C.I. −0.28 to 0.61). The effect of gender was analysed and the mean size of uncemented acetabular component implanted in males for the surface replacement group was not significantly different (p = 0.06) to the hybrid primary total hip replacement group. However the mean size of uncemented acetabular component in females for the surface replacement group was significantly smaller (p = 0.016) compared to the primary total hip replacement group. We conclude hip resurfacing is not bone sacrificing on the acetabular bone stock and can be bone conserving for females.