Investigation of imaging magnification in radiostereometric analysis

Phantom validation of a novel RSA-based impingement metric to assess component-on-component impingement risk

Component-on-component impingement in total hip arthroplasty may lead to post-operative complications including dislocation. Despite numerous clinical studies focusing on reducing this risk, assessment methods remain limited to qualitative radiography, finite element analysis, and cadaver studies. There is a need for more precise measurements of impingement in the research setting. We aimed to validate a novel RSA-based impingement metric to measure component-on-component impingement in vivo. A phantom experiment of a standard metal-on-polyethylene total hip system was performed. RSA examinations were performed as typical for a traditional weight-bearing RSA exam for large joints. The phantom was placed in 10 possible impinged positions and one neutral position. Double exposure radiographs were taken to measure repeatability. The closest distance between the skirt of the head and the inner circumference of the acetabular cup liner was measured to assess impingement risk. Distances between the closest point of the hood to the edge of the cup in 10 impinged positions ranged from 0.05 to 1.03 mm, with the average being 0.67 mm. In the neutral position, the distance measured is 11.02 mm. Excellent repeatability was observed, with a standard deviation of 0.03 mm with an r value of 0.09. A validated RSA-based risk metric was established to evaluate in vivo hip impingement. A 1 mm threshold may be proposed to define impingement where distances approaching 1.00 mm are at a greater risk of impingement. This simplified metric holds promise for upcoming clinical studies on component-on-component impingement.

Radiostereometric Analysis Permits In Vivo Measurement of Very Small Levels of Wear in TKA

BACKGROUND

Revision of TKA as a result of polyethylene wear is decreasing, but long-term wear performance of polyethylene is still a topic of interest to surgeons and device manufacturers seeking to improve longevity. Measuring wear of modern, wear-resistant implants has been described using radiostereometric analysis (RSA). Performing in vivo measurements would establish whether implant retrieval studies are representative of wear in well-performing knees.

QUESTIONS/PURPOSES

For a single knee implant system, we sought to determine (1) the linear wear rate using RSA; (2) the association between demographic factors and wear rate; and (3) the association between limb alignment and wear rate.

METHODS

A total of 49 patients with a minimum followup of 10 years (median, 12 years; range, 10-20 years) were retrospectively selected. During the examined period, 4082 TKAs were performed of which 2085 were the implant examined in this study. There were 71 of these patients who met the criteria including an available full-leg radiograph postoperatively, and 34 of these patients returned for examination along with 15 additional from a separate RSA study that also met the criteria. All patients received a posterior-stabilized, cobalt-chromium-on-conventional polyethylene total knee implant from a single implant system, which was the most commonly used at our institution at the time. Patients underwent standing RSA examinations from 0° to 120° of flexion at a single time point without the use of marker beads. Linear wear rates (including creep) were measured based on intersections between the femoral component and tibial insert models. Associations between wear and patient age at surgery, sex, height, weight, body mass index, tibial insert size, and limb alignment were examined.

RESULTS

Using the maximum linear wear rate from any flexion angle, the lateral rate was 0.047 mm/year (interquartile range [IQR], 0.034-0.066 mm/year) and the medial rate was 0.052 mm/year (IQR, 0.040-0.069 mm/year). Using the median of the linear wear rates across all flexion angles, the lateral rate was 0.027 mm/year (IQR, 0.017-0.046 mm/year) and the medial rate was 0.038 mm/year (IQR, 0.022-0.054 mm/year). This rate for males was 0.049 mm/year medially (IQR, 0.042-0.077 mm/year) and 0.032 mm/year laterally (IQR, 0.026-0.059 mm/year), and for females was 0.027 mm/year medially (0.016-0.039 mm/year) and 0.020 mm/year laterally (IQR, 0.013-0.032 mm/year). The wear rate for males was greater medially (difference = 0.022 mm/year, p < 0.001) and laterally (difference = 0.012 mm/year, p = 0.008). There were associations between greater wear and increasing height (ρ = 0.48, p < 0.001 medially and ρ = 0.30, p = 0.04 laterally), decreasing body mass index (ρ = -0.31, p = 0.03 medially), and greater implant size (ρ = 0.34, p = 0.02 medially). Increasingly varus leg alignment was associated with greater medial wear (ρ = 0.33, p = 0.02).

CONCLUSIONS

Greater wear rates were associated with demographic factors and leg alignment. Further RSA wear studies of other modern implant systems would provide complementary information to retrieval studies and valuable data on wear resistance.

CLINICAL RELEVANCE

Good wear resistance was demonstrated by well-performing implants in patients at long-term followup with wear magnitudes in agreement with reported values from retrieval studies.