Introduction: Femoro-acetabular impingement (FAI) is a common source of impaired motion of the hip, often attributed to the presence of an aspherical femoral head and reduced concavity of the anterior head/neck junction. However, other types of femoral deformity, including posterior slip, retroversion, and neck enlargement, can also limit hip motion. This study was performed to establish whether the “cam” impinging femur is a unique entity with a single deformity of the head/neck junction or is part of a multi-component continuum of femoral dysmorphia.

Materials and Methods: Computer models of 71 femora (28 normal and 43 “cam” impinging) were prepared from CT scans. Morphologic parameters describing the shape and dimensions of the head, neck, and medullary canal were calculated for each specimen. The anteversion angle, alpha angle of Notzli, beta angle of Beaulé, and normalized anterior heads offset were also calculated. Average dimensions were compared between the normal and impinging femora. A dimensionless model of the femoral neck was also generated to determine whether there is an inherent difference in the shape of the femoral neck in cam impinging and normal femora, independent of any differences in specimen size.

Results: Compared to the normal controls, the impinging femora had wider necks (AP: 15.2 vs 13.3 mm, p< 0.0001), larger heads (diameter: 48.3mm vs 46.0mm, p=0.032) and decreased head/neck ratios (1.60 vs 1.74, p=0.0002). However, there was no difference in neck/shaft angle (125.7° vs 126.5°, p=0.582) or anteversion angle (8.70 vs 8.44°, p=0.866). Most significantly, 53% of impinging femora also had a significant posterior slip (> 2mm), compared to only 14% of normal controls. Average head displacements for the two groups were: FAI: 1.93mm vs Normals: 0.78mm (p< 0.0001). Shape indices derived from individual dimensionless models showed slight AP widening of the abnormal femora (ap/ml ratio: 1.10 abnormal vs. 1.07 normal).

Conclusions: The CAM impinging femur has many abnormalities apart from the morphology of the head/neck junction. These femora have increased neck width and head/neck ratio, a smaller spherical bearing surface, and reduced neck offset from the medullary canal. Moreover, the presence of posterior head displacement and reduced anteversion should be appreciated when assessing treatment options, as surgical treatment limited to localized re-contouring of the head–neck profile may fail to address significant components of the underlying abnormality.

Introduction: The ten-year survivorship of Oxford Unicompartmental Knee Arthroplasty (OUKA) has ranged from 98% in the hands of the developers to only 82–90% in reports from independent centers and national registries. This study was performed to investigate the effects of surgeon training and correct patient selection on the expected outcome of this procedure.

Methods: We created a computer-simulated joint registry consisting of 20 surgeons who performed OUKA on 1,000 patients. Mathematical models of the patient and surgeon populations and corresponding hazard functions were formulated using data from the Swedish and Australian joint registries. The long-term survivorship of UKA was assumed to average 94% at 10 years and was modeled as the product of hazard functions quantifying risk factors under the surgeon’s control, risk factors presented by the patient, and the inherent revision risk of the procedure. We performed four simulations looking at the effect of surgeon training by pairing surgeons and patients based on surgeon experience and patient risk factors.

Results: When experienced surgeons (> 40 cases) performed OUKA on low risk patients (bottom quintile), the revision rate dropped from 6.0% to 4.5%. The same surgeons had a revision rate of 7.5% when assigned to the highest risk patient group (top quintile). Conversely, when the least experienced surgeons (< 10 cases) selected the least fit patients, the revision rate increased from 6% to 8.25%. However, when these surgeons were assigned to the lowest risk group, only 5.25% of patients were revised. Taken simultaneously, these results indicate that the overall revision rate of this procedure can vary between 4.5% to 8.25%, depending upon the experience of the surgeon and the patients selected.

Conclusions:

Mathematical models of patients and surgeons can be built using joint registry data. These models can then be used in a computer simulation yielding results comparable to what has been reported in the literature.

Polyethylene wear debris in TKA arises from several sources, including the tibiofemoral articulation and the interface between the backside surface of the tibial insert and the metal tibial tray. In this study we identify a new source of abrasive damage to the polyethylene bearing surface: impingement of resected bony surfaces, osteophytes and overhanging acrylic cement on the tibial bearing surface during joint motion.

One hundred forty-eight tibial components of 24 different designs in a retrieval collection were examined. A digital image of the articular surface of each insert was recorded. The presence, location and projected area of abrasive wear to the non-articulating edges of the insert were assessed using image analysis software.

Significant abrasive wear was observed in 24% of the retrievals with cemented femoral components and 9% from non-cemented components. Of the retrievals exhibiting this abrasive wear mode, 46% experienced multiple site damage. The average damage area for each individual abrasive scar was 78±11mm^2. Within the group of worn inserts, the abrasive scars were seen with a frequency of 69% on the extreme medial edge, 19% on the extreme lateral edge, 38% on the posterior-medial edge and 23% on the posterior-lateral edge. In posterior stabilized components with an open femoral box design, scarring of the superior surface of the tibial post was also observed. This proposed mode of damage was confirmed with several retrieved femoral components containing either fixed cement protruding from the posterior condyles, from the medial and lateral edges or osteophytes embedded in the posterior capsule. The corresponding inserts exhibited significant abrasive scarring at those locations.

We have observed a previously unrecognized source of polyethylene damage resulting in gouging, abrasion and severe localized damage in cemented and uncemented total knee replacement. Clearly, acrylic cement, in bulk or particulate form, often contributes to severe damage of the tibial surface and improvements to instruments and techniques for cementing are needed to prevent this wear mechanism.

Wear of the underside of modular tibial inserts (“backside wear”) has been reported by several authors. However, the actual volume of material lost through wear of the backside surface has not been quantified. This study reports the results of computerized measurements of tibial inserts of one design known to have a high incidence of backside wear in situ.

A series of retrieved TKA components of one design (AMK, Depuy) with evidence of severe backside wear and extrusions of the polyethylene insert were examined. The three-dimensional surface profile of the backside of each insert was digitized and reconstructed with CAD software (UniGraphics). The volume of material removed was calculated from the volume between the worn backside surface and an “initial” surface defined by unworn areas.

Computer reconstructions showed that in all retrievals, the unworn surface of the remaining pegs, the rim of material extruded over the medial edge and unworn surfaces on the anterior-lateral edge all lie in a single plane. This demonstrates that the “pegs” present on the backside of these inserts correspond to residual, unworn protrusions remaining on each retrieved component and do not represent cold flow extrusions through the base plate holes. The average volume of material lost due to backside wear was 608mm^3 ± 339mm^3 (range:80–1599 mm^3). This corresponds to an average loss of 569mg and an average linear wear rate of 103mg/year, based on the time in situ for each implant.

The volume of material removed due to backside wear is significant and is of a magnitude large enough to generate osteolysis. Our results indicate that the appearance of pegs on the underside of components with screw holes on the baseplate are not due to creep, but instead are due to severe wear of the insert. The mechanisms of material removed due to pitting and burnishing actually produce debris of a size more damaging in terms of osteolysis than wear at the articulating surface making it clear that significant improvements in implant design are needed to prevent backside wear and osteolysis.