Sclerosing Bone Dysplasias (Osteopetrosis, Pycnodysostosis, Osteopathia Striata, Sclerosteosis/Van Buchem)

Haematopoietic stem-cell transplantation is the only curative treatment for malignant infantile osteopetrosis - it supplies donor-derived osteoclasts. Two modern series give the figures the recommendation needs.

• Survival is now good. A prospective series of 19 children (median age 17 months) using busulfan-based myeloablative conditioning reported 2-year overall survival 84.2% and disease-free survival 73.7%; the three deaths were from infection, graft-versus-host disease and disease progression. A later series of 31 children using reduced-intensity conditioning (fludarabine, treosulfan, thiotepa and antithymocyte globulin) reported 100% overall survival - but with a median follow-up of only 363 days (range 74-1891), so that figure describes early survival, not cure, and should not be quoted as a long-term result.
• The morbidity is substantial. In that 31-patient series: acute graft-versus-host disease in 38.7%, hypercalcaemia in 25.8%, and viral reactivation in almost 80%, with two cases of Epstein-Barr-virus-driven post-transplant lymphoproliferative disease. All were successfully treated, but this is not a low-risk intervention.
• THE KEY POINT FOR THE EXAM: transplant does not reverse established damage. In the 19-patient series, visual acuity improved in only 2 patients and conductive hearing loss partially reversed in only 4. Cranial-nerve damage from compression is largely irreversible, which is precisely why both groups conclude that transplantation should be performed as early as possible after diagnosis, before severe sequelae develop. The urgency is about preserving sight and hearing, not only about marrow failure.
• Partial engraftment may be enough. Three children with mixed chimerism (45%, 45% and 70% donor cells) remained disease-free, suggesting full donor chimerism is not required to resolve the disease.

• Endochondral group. Osteopetrosis - the osteoclast cannot acidify the resorption lacuna: TCIRG1 (the proton-pump a3 subunit; commonest severe autosomal-recessive form), CLCN7 (the chloride channel; AR-intermediate and the AD Albers-Schonberg type), and CAII (carbonic anhydrase II; the syndrome of osteopetrosis + renal tubular acidosis + cerebral calcification). Pycnodysostosis - CTSK (cathepsin K, the protease that degrades collagen after demineralisation). Osteopoikilosis (and Buschke-Ollendorff) - LEMD3. Osteopathia striata - WTX/AMER1 (X-linked, with cranial sclerosis).
• Intramembranous group. Camurati-Engelmann - an activating TGFB1 mutation (enhanced TGF-beta signalling). Sclerosteosis / Van Buchem - loss of SOST/sclerostin (a Wnt antagonist), so unopposed Wnt drives overgrowth.
• Mixed. Melorheostosis - LEMD3 or somatic MAP2K1 mutations.
• The unifying axis. Density comes either from failed resorption (osteopetrosis, pycnodysostosis) or enhanced formation (the sclerostin/Wnt disorders) - which is why anti-sclerostin (romosozumab) was developed from the Van Buchem/sclerosteosis biology.

• Osteopetrosis: bone-in-bone (endobone), Erlenmeyer-flask metaphyses, and the 'rugger-jersey' / sandwich vertebra (dense superior and inferior end-plate bands) - with transverse 'chalk-stick' fractures.
• Pycnodysostosis: acro-osteolysis of the distal phalanges, wormian bones / open fontanelle, and an obtuse (increased) mandibular angle.
• Osteopoikilosis: multiple small round periarticular densities (symmetric); osteopathia striata: linear metaphyseal striations.
• Camurati-Engelmann: symmetric diaphyseal cortical thickening (sparing the epiphyses); melorheostosis: 'flowing candle-wax' hyperostosis along one side of a bone (monomelic/asymmetric).

Be honest about the evidence base here, because it is weaker than the confident tone of most teaching. No cohort study of fracture fixation in osteopetrosis exists. The literature is case reports and small reviews, and it is subject to obvious publication bias toward successes. That has two consequences for a viva answer.

• Union is achievable, and "nonunion is inevitable" overstates it. Reported cases of open reduction and internal fixation have united, one author concluding that treated patients showed union ability similar to normal. But union can be slow - 12 months in one reported subtrochanteric fracture - so judge fixation stability against a longer timeline than usual and do not mistake slow union for failure.
• The documented hazards are TECHNICAL rather than biological. Reported difficulties include drill-bit breakage and heat generation, screws breaking and being left in the canal, difficulty identifying the medullary canal at all, very difficult hardware removal, and intraoperative fracture during arthroplasty (an acetabular fracture in one conversion hip replacement). Refracture and displacement after fixation are recurring themes.
• Failed fixation compounds itself. One reported patient had sustained four previous femoral fractures inadequately treated before definitive surgery, and conversion total hip arthroplasty after failed fixation was described as substantially more complex and unpredictable. The first operation is the one that matters most.
• Practical implications. Plan preoperatively with CT, have multiple sharp drill bits and broken-screw removal equipment available, pre-drill and irrigate to limit thermal necrosis, accept longer operating times, and anticipate that intramedullary options may be impossible if no canal can be found.