Abstract
Background
Load transfer to the bone is believed to be more physiological around the short stem in total hip arthroplasty (THA). However, we found unusual bony remodeling around the shortened tapered femoral stem.
Methods
Among 121 consecutive THA using the same shortened tapered stem, 25 hips were excluded because the lateral cortex was already disturbed by previous surgery on the proximal femur. Sixteen hips were also excluded either because direct measurement was unavailable due to improperly taken final radiographs (n=9) or the patient was lost to follow-up (n=7).80 THAs were finally enrolled. Radiographic measurements were made using anteroposterior (AP) radiographs taken immediately and at 2 years after surgery. The thickness of the lateral cortex at the level of the distal end of the coated surface and at 10, 20, 30, and 40 mm proximal to it were measured. Variables for detecting the causative factors were age, gender, BMI, proximal femoral geometry, whether the surgery was done to dominant side, diagnosis leading to surgery, size and offset of the stem, articulation, alignments and operative time.
Results
The mean thickness of the lateral cortical bone measured at 10, 20, 30, and 40 mm above the tip of the proximal coating significantly decreased over the course of the 2 years (P<0.001 each). In 46 cases (57.5%), this presented as an intra-cortical osteolytic line (IOL). The mean thickness of the lateral cortex was reduced by more than 10% in 51 cases (63.8%). Sixty-one cases (76.3%) had either an IOL or showed a reduction in lateral cortical thickness greater than 10%. In 37 cases (46.3%), the lateral cortical thickness decreased by more than 20%. The risk of a mean reduction >20% was related to an increased operating time (odds ratio [OR] = 0.981; 0.966 < 95% confidence interval [CI] < 0.996) and lower body mass index (BMI) (OR = 1.216; 1.043 < 95% CI < 1.417). There was one periprosthetic fracture through the atrophied lateral cortex in one patient, The mean reduction of lateral cortex in this patient was 33.2%
Conclusion
Even with THA using a shortened stem, high incidence of proximal stress shielding was noted in the form of lateral cortical atrophy, especially for the patient with low BMI.
