Inaccuracies in the Use of Magnification Markers in Digital Hip Radiographs.

BACKGROUND: With the ubiquity of digital radiographs, the use of digital templating for arthroplasty has become commonplace. Although improved accuracy with digital radiographs and magnification markers is assumed, it has not been shown. QUESTIONS/PURPOSES: We wanted to (1) evaluate the accuracy of magnification markers in estimating the magnification of the true hip and (2) determine if the use of magnification markers improves on older techniques of assuming a magnification of 20% for all patients. METHODS: Between April 2013 and September 2013 we collected 100 AP pelvis radiographs of patients who had a THA prosthesis in situ and a magnification marker placed per the manufacturer's instructions. Radiographs seen during our standard radiographic review process, which met our inclusion criteria (AP pelvic view that included a well-positioned and observed magnification marker, and a prior total hip replacement with a known femoral head size), were included in the analysis. We then used OrthoView(TM) software program to calculate magnification of the radiograph using the magnification marker (measured magnification) and the femoral head of known size (true magnification). RESULTS: The mean true magnification using the femoral head was 21% (SD, 2%). The mean magnification using the marker was 15% (SD, 5%). The 95% CI for the mean difference between the two measurements was 6% to 7% (p < 0.001). The use of a magnification marker to estimate magnification at the level of the hip using standard radiographic techniques was shown in this study to routinely underestimate the magnification of the radiograph using an arthroplasty femoral head of known diameter as the reference. If we assume a magnification of 20%, this more closely approximated the true magnification routinely. With this assumption, we were within 2% magnification in 64 of the 100 hips and off by 4% or more in only four hips. In contrast, using the magnification marker we were within 2% of true magnification in only 20 hips and were off by 4% or more in 59 hips. CONCLUSION: We found the use of a magnification marker with digital radiographs for preoperative templating to be generally inaccurate, with a mean error of 6% and range from -5% to 15%. Additionally, these data suggest that the use of a magnification marker while taking preoperative radiographs of the hip may be unnecessary, as simply setting the software to assume a 20% magnification actually was more accurate. LEVEL OF EVIDENCE: Level III, diagnostic study.

A comparison of two implant systems in restoration of hip geometry in arthroplasty.

BACKGROUND: Restoration of hip offset and leg length during THA is often limited by available implant geometries. The recent introduction of femoral components with a modular junction at the base of the neck (two modular junction components) has expanded the options to restore femoral offset and leg length. QUESTIONS/PURPOSES: We asked (1) whether a femoral component with two modular junctions would predict by templating more frequent restoration of preoperative offset and leg length abnormalities than one with single modular junctions; and (2) how our use of these options compared with national sales data. PATIENTS AND METHODS: We retrospectively reviewed the preoperative templating data in 100 primary THAs using single modular junction implants with only a neutral version stem and 100 THAs using two modular junction implants. We compared the frequency with which the desired leg length and offset were completely restored by preoperative templating in the two groups. RESULTS: Offset and leg lengths were restored to within 1 mm in 85% of cases with two modular junction implants and 60% of cases with single modular junction implants. An anteverted or a retroverted neck was used in 25% of cases with the two modular junction stems. The national sales data revealed femoral neck components with version were used in 28% of cases. CONCLUSIONS: The use of a femoral component with two modular junctions resulted in more frequent ability to restore femoral offset and leg length than a single modular junction. The advantage of clinical flexibility should be tempered by the potential concerns of prosthetic mechanical failure (which has been reported in another implant system with two modular junctions), increased third-body wear and corrosive debris, and increased prosthetic cost. LEVEL OF EVIDENCE: Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Polyethylene exchange in a second-generation cementless acetabular component.

Some have suggested that isolated polyethylene exchange in a well-fixed Harris-Galante II acetabular component (Zimmer, Warsaw, Ind) necessitates cementing the liner or complete revision because the locking mechanism is suboptimal. We reviewed 29 hip revisions during which the polyethylene was exchanged using the native locking mechanism. Mean follow-up was 5.1 years (2-13 years). Of the 29 patients, one had a disengagement of the revision polyethylene at 2.5 years. At the time of this patient's original revision, one of the tines was fractured, but a direct exchange was performed. There were 4 other revisions (one for loosening and 3 for instability). There were no other complications attributable to the direct polyethylene exchange and no further reoperations. This series suggests that polyethylene exchange with the Harris-Galante II prosthesis can be performed safely using the native locking mechanism in the absence of fractured tines.

Acetabular loosening using an extended offset polyethylene liner.

UNLABELLED: The use of extended offset femoral components and acetabular liners helps restore preoperative offset during hip arthroplasty. We report a relatively high acetabular component aseptic loosening rate with the use of offset polyethylene liners. We reviewed 1919 primary and 346 revision total hip arthroplasties (THAs). A 7-mm offset acetabular liner was used in 120 of the primary and 100 of the revision THAs. The aseptic loosening rate in the primary THA group was 0.12% in the standard offset and 4.2% in the extended offset groups at a minimum of 2 years (mean, 3.6 years; range, 2-9 years) followup. The aseptic loosening rate in the revision group was 1.7% in the standard and 7% in the extended offset groups at a mean of 4 years (range, 2-9 years) followup. Although extended offset acetabular liners help restore hip offset, torsional force applied to the implant-bone interface may have a detrimental effect on fixation. We found a relatively high failure rate in our primary and revision acetabular components used with an offset liner. LEVEL OF EVIDENCE: Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.