Does Degenerative Lumbar Spine Disease Influence Femoroacetabular Flexion in Patients Undergoing Total Hip Arthroplasty?
Venue: Clinical orthopaedics and related research

BackgroundSitting pelvic tilt dictates the proximity of the rim of the acetabulum to the proximal femur and, therefore, the risk of impingement in patients undergoing total hip arthroplasty (THA) Sitting position is achieved through a combination of lumbar spine segmental motions and/or femoroacetabular articular motion in the lumbar pelvic femoral complex. Multilevel degenerative disc disease (DDD) may limit spine flexion and therefore increase femoroacetabular flexion in patients having THAs, but this has not been well characterized. Therefore, we measured standing and sitting lumbar pelvic femoral alignment in patients with radiographic signs of DDD and in patients with no radiographic signs of spine arthrosis. Is there a difference in standing and sitting lumbar pelvic femoral alignment before surgery among patients undergoing THA who have no radiographic signs of spine arthrosis compared with those with preexisting lumbar DDD? (2) Do patients with lumbar DDD experience less spine flexion moving from a standing to a sitting position and therefore compensate with more femoroacetabular flexion compared with patients who have no radiographic signs of arthrosis?MethodsThree hundred twenty five patients undergoing primary THA had preoperative low dose EOS spine to ankle lateral radiographs in standing and sitting positions. Eighty three patients were excluded from this study for scoliosis (39 patients) spondylolysis (15 patients) not having five lumbar vertebrae (7 patients) surgical or disease fusion (11 patients) or poor image quality attributable to high BMI (11 patients) In the remaining 242 of 325 patients (75% two observers categorized the lumbar spine as either without radiographic arthrosis or having DDD based on defined radiographic criteria. Sacral slope, lumbar lordosis, and proximal femur angles were measured, and these angles were used to calculate lumbar spine flexion and femoroacetabular flexion in standing and sitting positions. Patients were aligned in a standardized sitting position so that their femurs were parallel to the floor to achieve approximately 90deg of apparent hip flexion.ResultsAfter controlling for age, sex, and BMI, we found patients with DDD spines had a mean of 5deg more posterior pelvic tilt (95% CI, 2deg to 8deg lower sacral slope angles; p 0.01) and 7deg less lumbar lordosis (95% CI, 10deg to 3deg; p 0.01) in the standing position compared with patients without radiographic arthrosis. However, in the sitting position, patients with DDD spines had 4deg less posterior pelvic tilt (95% CI, 1deg 7deg higher sacral slope angles; p 0.02) From standing to sitting position, patients with DDD spines experienced 10deg less spine flexion (95% CI, 14deg to 7deg; p 0.01) and 10deg more femoroacetabular flexion (95% CI, 6deg to 14deg; p 0.01).ConclusionsMost patients undergoing THA sit in a similar range of pelvic tilt, with a small mean difference in pelvic tilt between patients with DDD spines and those without radiographic arthrosis. However, in general, the mechanism by which patients with DDD of the lumbar spine achieve sitting differs from those without spine arthrosis with less spine flexion and more femoroacetabular flexion.Clinical RelevanceWhen planning THA, it may be important to consider which patients sit with less posterior pelvic tilt and those who rotate their pelvises forward to achieve a sitting position, as both mechanisms will limit or reduce the functional anteversion of the acetabular component in a patient with a THA. Our study provides some additional perspective on normal relationships between pelvic tilt and femoroacetabular flexion, but further research might better characterize this relationship in outliers and the possible implications for posterior instability after THA.