Review Question - QID 4814

The clinical presentation is consistent with osteopetrosis. Osteopetrosis is associated with appendicular fractures, osteomyelitis, cranial nerve palsies, and coxa vara. Osteopetrosis is not associated with vertebral compression fractures.

Osteopetrosis is caused by genetic mutations affecting acidification of Howship’s lacuna. Proton pump mutations are most common (60%), followed by mutations in the chloride channel CLCN7 (12%) and mutations in carbonic anhydrase II (<5%). The difference mutations drive the clinical phenotype and lead to the three different types of osteopetrosis including: malignant infantile (AR), intermediate (AR) and benign type II (AD).

Landa et al. reviewed surgical management of osteopetrosis. For fractures, they recommend pre-drilling pin and screw holes, and using nails instead of plates. For arthroplasty, they recommend opening the medullary canal with drill bits of increasing diameter, and short femoral stems (THA).

Dietz et al. reviewed genetic disorders with orthopaedic manifestations. The gene for autosomal recessive osteopetrosis is found on 1p21-13.

Figure A is an AP radiograph of the chest and upper extremities demonstrating marked generalized bone sclerosis. Figure B is an AP radiograph of the pelvis and lower extremities showing Erlenmeyer flask femora and incidental inguinal hernia. Illustration A shows endobone (bone-within-bone) formation. Illustration B shows sandwich vertebrae (rugger jersey spine).

Incorrect Answers:
Answer 1: They are at increased incidence of appendicular fractures (lifelong prevalence, 40-60%). This is because calcified cartilage and thick trabeculae remain in the diaphysis, and cartilage has inferior mechanical properties to bone.
Answer 2: There is an increased incidence of osteomyelitis. This is due to the lack of marrow vascularity in osteopetrotic bone and impairment of white cell function. This is of concern after fracture fixation and arthroplasty.
Answer 3: Cranial nerve palsies arise because of compression of cranial nerves by overgrowth of the skull foramina. The optic and auditory nerves are most affected (optic > auditory > trigeminal > facial)
Answer 4: There is increased incidence of coxa vara. They arise from nonunion of femoral neck fractures, or stress-induced microfractures in the femoral neck.