A patient with an extensively porous-coated cylindrical uncemented femoral stem fixed rigidly in the femoral diaphysis develops progressive rarefaction of the calcar region on follow-up radiographs. Which mechanism explains this pattern of bone loss?
- A Distal load transfer bypassing the proximal femur, causing stress shielding in Gruen zones 1 and 7 ✓
- B Wear particle driven osteolysis tracking along the cement-bone interface
- C Thermal necrosis of the medial femoral neck from cement polymerisation
- D Immune reaction to titanium alloy degranulating osteoclast precursors
Correct answer: A. Distal load transfer bypassing the proximal femur, causing stress shielding in Gruen zones 1 and 7
Explanation
Extensively coated stiff stems achieve fixation in the diaphysis, transferring loads distally and offloading the proximal femur. Bone follows Wolff's law, so reduced stress in the proximal medial cortex, Gruen zones 1 and 7 including the calcar, leads to adaptive resorption seen as stress shielding. Wear particle osteolysis produces focal lytic lesions rather than smooth regional rarefaction, and cement-related thermal necrosis is irrelevant to an uncemented implant.
Reference: Miller's Review of Orthopaedics, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.