Common femoral stem implants too big for Chinese patients

Femoral stem implants conventionally used in hip arthroplasty are generally unsuitable for Chinese patients, according to a recent Singapore study.

“This study … is the first to show that conventional Western implants may not be suitable for the Chinese population. There are notable differences between the medial femoral offset (MFO) of femoral stem implants used and the native MFO of [patients]. This paves the way for further prospective studies to evaluate such differences,” the researchers said.

Radiographic data from 300 patients who had undergone hip arthroplasties were retrospectively evaluated. Their proximal femur morphological measurements were obtained and compared to those of conventionally used femoral stems. Those with malrotated femurs were excluded to ensure more accurate measurements and comparisons.

The mean femoral head diameter among participants was 43.5±3.29 mm while the average neck shaft angle was 130.8±3.96°. The mean medial and vertical femoral offsets were 33.5±3.68 and 48.7±5.16 mm, respectively. [Malays Orthop J 2021;15:101-106]

Sex differences were observed for femoral head diameter (men vs women: 47.3±3.33 vs 42.5±2.39 mm), MFO (36.1±3.94 vs 32.8±3.27 mm), and vertical femoral offset (53.0±5.4 vs 47.5±4.40 mm; p<0.0001 for all).

Likewise, significant differences in femoral measurements were detected according to age groups (60–75 vs 75–90 years): femoral head diameter (47.5±3.33 vs 42.5±2.39 mm), MFO (36.1±3.94 vs 32.8±3.27 mm), and vertical femoral offset (53.0±5.4 vs 47.5±4.4 mm; p<0.0001 for all). Neck shaft angle was comparable across sexes and age groups.

Crucial differences in morphological measurements were identified to make commonly available implants unsuitable for use. In particular, 22.6 percent of women and 3 percent of men had MFOs lower than 30 mm, which is the smallest size offered by all examined implant brands.

Moreover, the average offset value in men was lower than the smallest available size offered by three implants. Women, in comparison, were below the size ranges of four brands.

“Our study shows that commonly used femoral stem implants for hip are unsuitable for the elderly Chinese hip fracture population with low medial femoral offsets … in particular for female patients,” the researchers said.

“For this subgroup of patients with an offset of less than 30mm, vertical offset may have to be reduced to maintain abductor tension which may result in limb length discrepancy. Therefore, we recommend a reduced offset implant for the Chinese population, especially in the wake of increasing incidence of osteoporosis and hip fractures in Asia,” they added.

Notable limitations of the study included the use of two-dimensional radiographic images, failure to include radiographic markers, and lack of analysis of functional outcomes due to its radiographic design. The findings are also largely only applicable to the Chinese population and may not be generalizable to other ethnicities. Future studies are needed to address these shortcomings.