Analysis of polyethylene wear in plain radiographs

Does Pelvic Orientation Influence Wear Measurement of the Acetabular Cup in Total Hip Arthroplasty—An Experimental Study

Roentgen stereophotogrammetric analysis (RSA) is the gold standard to detect in vivo material wear of the bearing couples in hip arthroplasty. Some surgical planning tools offer the opportunity to detect wear by using standard a.p. radiographs (2Dwear), whilst RSA (3Dwear) needs a special radiological setup. The aims of this study are to prove the interchangeable applicability of a 2Dwear approach next to RSA and to assess the influence of different pelvic positions on measurement outcomes. An implant-bone model was used to mimic three different wear scenarios in seven pelvic-femur alignment positions. RSA and a.p. radiographs of the reference and a follow-up (simulated wear) pose were acquired. Accuracy and precision were worse for the 2Dwear approach (0.206 mm; 0.159 mm) in comparison to the 3Dwear approach (0.043 mm; 0.017 mm). Changing the pelvic position significantly influenced the 2Dwear results (4 of 7, p < 0.05), whilst 3Dwear results showed almost no change. The 3Dwear is superior to the 2Dwear approach, as it is less susceptible to changes in pelvic position. However, the results suggest that a 2Dwear approach may be an alternative method if the wear present is in the range of 100–500 µm and a.p. radiographs are available with the pelvis projected in a neutral position.

Dimensional analysis of total hip arthroplasty polyethylenes

Objective

This paper performs a dimensional analysis of different total hip arthroplasty polyethylenes, cemented and non-cemented, Brazilian made and imported.

Methods

It was considered acetabular components with 50 mm for the 28 mm femoral heads. Dimensional analysis was performed on a 3D coordinate Carl-Zeiss robotic device. Polyethylene thickness and its external measurements (maximum diameter and diameter for the femoral head) were measured.

Results

The minimum thickness of the polyethylene was guaranteed on all tested components. The thickness of cemented acetabular varied from 19.185 mm to 25.358 mm, while the thickness of the non-cemented acetabular varied from 12.451 mm to 19.232 mm. The thickness was 27.96% lower in non-cemented acetabular components. With respect to the polyethylene acetabular cavity that receives the femoral head, all internal diameters exhibit at least 28 mm. In relation to the maximum outer diameter of the polyethylene, only one cemented acetabular component reached 50 mm in diameter.

Conclusions

There are large differences in measurements between brands and models analyzed. Cementless acetabular components have the smaller thickness. The diameters of non-cemented acetabular were also lower than those cemented at the expense of their need to insert into the metal-back.

Superior 11-year survival but higher polyethylene wear of hydroxyapatite-coated Mallory-Head cups

Hydroxyapatite (HA) coating of implant surfaces is believed to improve longevity of uncemented total hip arthroplasty (THA). However; it is speculated that HA particles may cause third body wear of the polyethylene (PE) liner with subsequent osteolysis and aseptic loosening of implants. We performed a retrospective comparison of two patient populations with cementless Mallory-Head acetabular components. The patients were identified in the Danish hip arthroplasty Registry (DHR); 77 patients had HA-coated cups and 73 patients had non-HA coated cups. Anteroposterior (AP) radiographs were analysed for PE wear, osteolysis was quantified and survival of the acetabular components was compared. The mean follow-up was 11 years. There were no cup revisions in the HA group and 7 cup revisions in the non-HA group (P<0.01). The reason for revision in all cases was aseptic loosening of both stem and cup. The amount of osteolysis was significantly increased around cups in the non-HA group. The two-dimensional linear PE wear-rate of 0.18 mm/year was higher (P<0.001) in the group with HA coated cups compared with 0.12 mm/year in the group with non-HA coated cups. The mean age was lower (P=0.001) in the HA group (57 years) compared with the non-HA group (63 years). After 11.1 years of follow-up non-HA coated cups had a higher revision rate. Increased PE wear with HA-coated cups did not have a negative influence on the revision rate, but may result in a need for revision surgery over time.

The final follow-up plain radiograph is sufficient for clinical evaluation of polyethylene wear in total hip arthroplasty. A study of validity and reliability

BACKGROUND AND PURPOSE

Radiostereometric analysis (RSA) is a highly accurate tool for assessment of polyethylene (PE) wear in total hip arthroplasty (THA); however, PE wear measurements in clinical studies are often limited to plain radiographs. We evaluated the agreement between PE wear measured with PolyWare software, which uses plain radiographs, and by model-based RSA, which uses stereo radiographs.

METHODS

Measurements of PE wear postoperatively and at final follow-up (after mean 6 years) on plain radiographs of 12 patients after cementless THA were evaluated with PolyWare software and the results were compared with those from RSA as the gold standard (Model-based RSA using elementary geometrical shape models; EGS-RSA). With PolyWare, we either used the final radiographic follow-up (PW1) only or both the postoperative follow-up and the final follow-up (PW2).

RESULTS

The 2D mean wear measured (in mm) was 0.80, 1.07, and 0.60 for the PW2, PW1, and RSA method. 2D intra-method repeatability was similar for PW1 and RSA with limits of agreement (LOAs, in mm) of ± 0.22, and ± 0.23, respectively. 2D inter-method concurrent validity was best between PW1 and EGS-RSA with LOAs of ± 0.55. For 2D linear wear measurements, the PW1 method had a clinical repeatability similar to that of RSA.

INTERPRETATION

PW1 is sufficient for retrospective determination of 2D wear from medium-term wear measurements above 0.5 mm, It alleviates the need for baseline plain radiographs, has a clinical precision similar to that of RSA, and is easy and inexpensive to use.