Patients with flatback deformity had lower simulated midpoint targets for combined anteversion, whereas patients with stiff spinopelvic mobility had a narrower acceptable anteversion range following total hip arthroplasty, according to a retrospective cohort study.
The study, published in JBJS Open Access, included 108 hips in 99 patients who underwent primary total hip arthroplasty and postoperative dynamic radiographic analysis. The researchers used previously collected three-dimensional-to-two-dimensional model-image registration data during sit-to-stand motion and postoperative range-of-motion simulation to estimate anteversion ranges that avoided prosthetic impingement. The Level III study did not evaluate clinical outcomes such as dislocation, revision, or patient-reported outcomes.
Total hip arthroplasty (THA) components were virtually positioned at their actual postoperative locations. Radiographic inclination was fixed at 40 degrees, and radiographic anteversion was adjusted in 1-degree increments. Combined anteversion (CA), defined as cup anteversion plus femoral stem anteversion, was calculated by adding femoral stem anteversion to acceptable radiographic anteversion values.
Flatback deformity was defined as pelvic incidence–lumbar lordosis mismatch greater than 10 degrees. Stiff spinopelvic mobility (SPM) was defined as a change in anterior pelvic plane angle of less than 10 degrees between seated and standing positions. Patients without flatback deformity, stiff SPM, hyperlordosis, or hypermobility were classified as having normal spinopelvic alignment.
Acceptable anteversion was defined as the range that satisfied all prespecified motion thresholds without prosthetic impingement in both maximal anterior pelvic tilt and standing pelvic tilt positions. These thresholds included at least 110 degrees of flexion, 30 degrees of extension, 30 degrees of internal rotation at 90 degrees of flexion, and 30 degrees of external rotation.
Compared with patients with normal spinopelvic alignment, patients with flatback deformity had a lower CA midpoint, at 41 degrees vs 47 degrees, and a lower maximum CA, at 50 degrees vs 57 degrees. Minimum CA and CA range did not statistically significantly differ between the groups.
The lower midpoint target may reflect the persistent posterior pelvic tilt seen in patients with flatback deformity, which can shift functional cup orientation and alter the anteversion needed to avoid impingement during movement.
Patients with stiff SPM had no statistically significant difference in CA midpoint compared with patients with normal alignment. However, their acceptable CA range was narrower, at 15 degrees vs 19 degrees. This narrower range may reflect reduced compensatory pelvic motion, requiring greater reliance on hip motion during functional activity and leaving less margin for component-positioning error.
Greater pelvic incidence–lumbar lordosis mismatch was associated with lower midpoint CA values, with the model estimating an approximately 2-degree decrease in target CA for every 10-degree increase in mismatch. Pelvic mobility was not associated with CA midpoint but was associated with the width of the acceptable CA range, with reduced mobility corresponding to a narrower simulated safe zone.
In multivariable modeling, greater femoral stem anteversion and greater pelvic incidence–lumbar lordosis mismatch were independently associated with lower midpoint radiographic anteversion. A narrower radiographic anteversion range was associated with female sex, higher body mass index, greater stem anteversion, smaller implant oscillation angle, smaller pelvic incidence–lumbar lordosis mismatch, and more limited pelvic mobility.
The researchers noted that the association between smaller pelvic incidence–lumbar lordosis mismatch and narrower radiographic anteversion range should be interpreted cautiously, because pelvic incidence–lumbar lordosis mismatch was not linearly correlated with anteversion range in unadjusted analysis. They suggested the multivariable finding may reflect interactions with pelvic mobility.
The findings may support more individualized acetabular component planning in patients with spinopelvic abnormalities. The researchers noted that approaches or implant designs that expand impingement-free range of motion, including computed tomography–based navigation, computer-assisted surgical systems, larger femoral heads, or dual-mobility liners, may be particularly relevant in patients with stiff SPM and narrower simulated safe zones.
Several limitations were noted. Spinopelvic alignment was assessed only postoperatively, motion analysis was limited to sit-to-stand movement, and the impingement model did not account for soft tissue or bone-to-bone contact. Cup inclination was also fixed at 40 degrees, and patient-reported outcomes were unavailable.
“[S]pinopelvic abnormalities significantly influence both the optimal target and the tolerance range of combined anteversion after THA,” wrote lead study researcher Toshiki Konishi, MD, of Kyushu University, and colleagues.
Disclosures: The study was supported by the Ogata Memorial Foundation, the Foundation for the Advancement of Clinical Medicine, and JSPS KAKENHI. The researchers reported no commercial associations posing conflicts of interest in connection with the submitted article. Verify online disclosure forms before publication.
Source: JB & JS Open Access