The external obturator footprint as a landmark in total hip arthroplasty through a direct anterior approach: a CT-based analysis

The External Obturator Footprint Is a Usable, Accurate, and Reliable Landmark for Stem Depth in Direct Anterior THA

BACKGROUND

Previous CT and cadaver studies have suggested that the external obturator footprint might be used as a landmark for stem depth in direct anterior THA. Instructions on where to template this structure with small variability in height have been developed but have not been tested in daily clinical practice.

QUESTIONS/PURPOSES

In this study we sought to investigate the (1) usability, (2) accuracy, and (3) reliability of the external obturator footprint as a landmark for stem depth in direct anterior THA.

METHODS

The distance between the superior border of the external obturator tendon and the shoulder of the stem was measured intraoperatively in all patients (n = 135) who underwent primary THA via a direct anterior approach performed by the senior author between November 2019 and October 2020. The landmark was considered useful when two of thre`e evaluators agreed that the intersection of the vertical line comprised of the lateral wall of the trochanteric fossa and the oblique line formed by the intertrochanteric crest was clearly visible on the preoperative planning radiograph, and when the landmark was furthermore identified with certainty during surgery. Accuracy was defined as the degree of agreement (categorical for thresholds of 2 and 5 mm, the latter representing the threshold for developing unphysiological gait parameters) between the intraoperative distance and radiographic distance as measured on intraoperative fluoroscopy images or postoperative radiographs, which were calibrated based on femoral head sizes in a software program commonly used for templating. Intrarater reliability was defined as the degree of agreement (categorical for thresholds of 1 mm, which we considered an acceptable measurement error) between the ratings of one observer, who measured the radiographic distance on two different occasions separated by a washout period of at least 2 weeks. Interrater reliability was defined as the degree of agreement (categorical for thresholds of 1 mm, which we considered an acceptable measurement error) between the ratings of three observers with varying levels of experience (a fellowship-trained hip surgeon, a hip surgery fellow, and a medical student).

RESULTS

The landmark was considered useful in 77% (104 of 135) of patients who underwent direct anterior THA based on the observations that the trochanteric fossa was clearly visible on the planning radiograph in 117 patients and that the tendon was identified with certainty during surgery in 118 patients. There was good-to-excellent accuracy (intraclass correlation coefficient 0.75-087), and intrarater reliability (ICC 0.99) and interrater reliability (ICC 0.99) were both excellent.

CONCLUSION

This clinical study showed that the external obturator footprint is a useful, accurate, and reliable landmark for stem depth in direct anterior THA.

CLINICAL RELEVANCE

The external obturator landmark allows the surgeon to position the stem within a range of the templated depth that is beneath the threshold for the development of unphysiological gait parameters. Although strictly speaking it was found useful in 77% of patients in this study, we found that this percentage of usability can easily be improved to around 90% by providing the radiology lab technician with instructions to correct external rotation of the foot during the taking of the planning radiograph. Future studies could compare the established (in)equality in leg length in patients using the external obturator landmark with computer-assisted surgery.

The trochanteric double contour is a valuable landmark for assessing femoral offset underestimation on standard radiographs: a retrospective study

Background

Inaccurate projection on standard pelvic radiographs leads to the underestimation of femoral offset—a critical determinant of postoperative hip function—during total hip arthroplasty (THA) templating. We noted that the posteromedial facet of the greater trochanter and piriformis fossa form a double contour on radiographs, which may be valuable in determining the risk of underestimating femoral offset. We evaluate whether projection errors can be predicted based on the double contour width.

Methods

Plain anteroposterior (AP) pelvic radiographs and magnetic resonance images (MRIs) of 64 adult hips were evaluated retrospectively. Apparent femoral offset, apparent femoral head diameter and double contour widths were evaluated from the radiographs. X-ray projection errors were estimated by comparison to the true neck length measured on MRIs after calibration to the femoral heads. Multivariate analysis with backward elimination was used to detect associations between the double contour width and radiographic projection errors. Femoral offset underestimation below 10% was considered acceptable for templating.

Results

The narrowest width of the double line between the femoral neck and piriformis fossa is significantly associated with projection error. When double line widths exceed 5 mm, the risk of projection error greater than 10% is significantly increased compared to narrower double lines, and the acceptability rate for templating drops below 80% (p = 0.02).

Conclusion

The double contour width is a potential landmark for excluding pelvic AP radiographs unsuitable for THA templating due to inaccurate femoral rotation.

Anatomical Mapping of the External Obturator Footprint: A Study In Cadavers with Implications for Direct Anterior THA

BACKGROUND

The external obturator footprint in the trochanteric fossa has been suggested as a potential landmark for stem depth in direct anterior THA. Its upper border can be visualized during surgical exposure of the femur. A recent study reported that the height of the tendon has little variability (6.4 ± 1.4 mm) as measured on CT scans and that the trochanteric fossa is consistently visible on conventional pelvic radiographs. However, it is unclear where exactly the footprint of this tendon should be templated during preoperative planning so that it can be useful intraoperatively.

QUESTIONS/PURPOSES

In this study, we sought: (1) to provide instructions on exactly where to template the external obturator footprint on a preoperative planning radiograph, and (2) to confirm the small variability in height of the external obturator footprint found on CT scans in a cadaver study.

METHODS

Two-dimensional (2-D) and three-dimensional (3-D) imaging was used to map the anatomy of the external obturator footprint. This dual approach was chosen because of their complementarity; conventional 2-D radiographs translate to clinical practice but 3-D navigation-based digitalization combined with CT allows for a better understanding of the cortical lines that comprise the outline of the trochanteric fossa. In 12 (four males, mean age 80 years, range 69 to 88) formalin-treated cadaveric lower extremities including the pelvis, the external obturator tendon was dissected, and the top and bottom end of its footprint marked with two small needles, and calibrated radiographs were taken. For another five (three males, mean age 75.7 years, range 61 to 91) fresh-frozen cadaveric lower extremities, including femoral reflective marker frames, CT scans were obtained and the exact location of the external obturator footprint was recorded using 3-D navigation-based digitalization. Qualitative analysis of both imaging modalities was used to develop instructions on where the external obturator footprint should be templated on a preoperative planning radiograph. Quantitative analysis of the dimensions of the external obturator footprint was performed.

RESULTS

The lowest point of the external obturator footprint was consistently found (± 1 mm) at the intersection of the vertical line comprised of the lateral wall of the trochanteric fossa and the oblique line formed by the intertrochanteric crest and therefore allows templating of this structure on the preoperative planning radiograph. The median (range) height of the footprint measured 6.4 mm and demonstrated small variability (4.7 to 7.6).

CONCLUSIONS

We suggest templating a 6.4-mm circle with its bottom on the intersection described above.

CLINICAL RELEVANCE

The distance between the templated shoulder of the stem and the top of the circle can be used intraoperatively for guidance. Discrepancy should lead to re-evaluation of stem depth and leg length. Future work will investigate the usability, validity, and reliability of the proposed methodology in daily clinical practice.