Review Question - QID 218208

Patients with adult hip dysplasia and chronic, complete hip dislocation (i.e. Crowe IV; Illustration A) often have notable (> 5cm) limb length discrepancies. In restoring limb length during total hip replacement, a subtrochanteric osteotomy is often required to prevent acute lengthening of the sciatic nerve and iatrogenic permanent foot drop (Answer 3).

The most common cause for premature osteoarthritis requiring arthroplasty (<50 years) is developmental dysplasia of the hip (DDH). Thankfully, routine screening on physical examination of newborns and infants has decreased the overall prevalence and severity of this disease in adulthood. However, severe cases left untreated result in notable dysfunction and lifestyle limitations. Total hip arthroplasty can restore functionality but is technically demanding, particularly in these severe cases. The most common challenges encountered on the acetabular side of the reconstruction include (1) a high-riding or chronically dislocated femoral head, (2) resultant deficient superolateral rim structural support, and (3) a small-diameter, shallow native acetabulum. These can be combated by obtaining adequate exposure, using smaller-than-usual implants, and utilizing standard acetabular shells with superolateral bone grafting, acetabular augments, or tantalum-backed revision-type acetabular shells. The goal in most cases should be to restore the hip center of rotation within the native, true acetabulum (i.e. low and medial cup positioning with small implants) rather than placing the cup high, lateral, and using oversized implants. In Crowe IV hips, a subtrochanteric osteotomy is recommended when restoring the native hip center of rotation results in acutely lengthening the lower extremity by >4cm (Illustration B).

Greber and colleagues provided a comprehensive review article highlighting technical pearls in treating DDH in the adult population. In addition to the aforementioned challenges along the acetabular side of reconstruction, the authors note the femoral side also requires special consideration. They discuss this includes (1) markedly increased native femoral anteversion, (2) a narrow diaphyseal canal, and (3) potentially over-lengthening the overall limb length with implantation. The authors discuss these challenges can be combated by utilizing modular implants where the surgeon can dial in specific anteversion, utilizing small diameter distally tapered stems, and performing a subtrochanteric osteotomy when acutely lengthening. They conclude that many complications associated with THA in the adult hip dysplasia patient may be mitigated with careful planning and surgical technique.

Rogers and colleagues provided a similar current concepts review article on performing THA for adult hip dysplasia. Similarly, the authors recommended placing the acetabular cup at the site of the true acetabulum rather than the pseudoacetabulum. They also discuss their technique of utilizing a trochanteric osteotomy to gain exposure, when needed, and the various bone grafting options to recreate the superolateral boney support that is often deficient in these patients. The authors conclude highlighting that clinicians and patients alike should be aware of the increased complication rate associated with performing THA in adult dysplasia, particularly dislocation and aseptic loosening.

Figure A demonstrates this patient’s AP pelvis radiographic imaging with 100% femoral head dislocation (i.e. Crowe IV dysplasia) and pseudoacetabulum formation within the iliac wing. Illustration A demonstrates the Crowe classification which is most commonly utilized when describing adult hip dysplasia. Illustration B demonstrates this patient’s post operative radiographs after performing THA. Importantly, a small (44mm) acetabular cup was placed at the level of the true, native acetabulum and was notably medialized to obtain adequate native boney support. Further, a subtrochanteric osteotomy was performed with placement of adjunctive cortical allograft (as recommended in the Rogers article) to optimize boney union.

Incorrect Answers:
Answer 1: A pseudoacetabulum forms when there is superolateral erosion of the native acetabulum from chronic femoral head subluxation or, in this case, gross dislocation. In general, it forms within the ilium which lacks appropriate structural support to receive an acetabular cup. While previously considered acceptable, placement of the acetabular cup in the pseudoacetabulum to avoid limb lengthening is fraught with complications to include aseptic loosening, cup malposition, and catastrophic failure from ilium fractures.
Answer 2: In general, there is inadequate structural boney support around the native acetabulum (secondary to superolateral rim wear) to provide desired circumferential press fit of a standard sized acetabular shell. The tendency is to medialize with larger sized reamers to obtain coverage, which can result in medial wall blow and/or iatrogenic reaming away of the anterior/posterior columns. For this reason, it is recommended to initially utilize extremely small reamers (38-44mm) and utilize a small final acetabular implant (<50mm) to obtain circumferential boney support.
Answer 4: Metal on metal-bearing surfaces have notable long-term complications secondary to metal debris produced with bearing surface wear and edge loading. Given most adult hip patients receive THA at an early age (<50 years), it is recommended to avoid utilizing metal-on-metal implants in these patients to avoid such complications. Ceramic on polyethylene bearing surfaces, in general, impart the longest survivorship with the fewest complications.
Answer 5: PMMA cement is exceptionally good at resisting compressive forces but is notably poor at resisting shearing forces. For this reason, cementing an acetabular cup into the pelvis is shown to have extremely high rates of aseptic loosening. With modern acetabular cups that can be manufactured with multiple screw holes or tantalum backing, cementing an acetabular cup is seldom recommended.