Fibrous Dysplasia — Shepherd’s Crook

Fibrous dysplasia (FD) is a benign skeletal condition in which normal medullary bone is replaced by abnormal fibrous tissue containing poorly mineralised woven bone trabeculae. It results from a post-zygotic activating mutation in the GNAS gene (encoding the Gs-alpha subunit of G-protein), which leads to unregulated osteoblastic differentiation and abnormal bone production. Despite being non-hereditary, fibrous dysplasia can cause significant skeletal deformity, pain, and pathological fractures, particularly in the proximal femur.

• GNAS gene mutation (Arg201His or Arg201Cys): activating somatic mutation; occurs post-zygotically — severity and distribution depends on timing of mutation during embryogenesis (mosaic distribution)
• Monostotic FD (70–80%): single bone involved; most commonly proximal femur, tibia, ribs, skull base, facial bones; discovered incidentally or after fracture
• Polyostotic FD (20–30%): multiple bones; typically unilateral (hemimelic) distribution; more severe deformity; diagnosed in childhood
• McCune-Albright syndrome: polyostotic FD + café-au-lait spots (coast of Maine borders) + endocrine hyperfunctioning (precocious puberty most common); café-au-lait spots in McCune-Albright are large, irregular, with jagged borders — distinct from neurofibromatosis spots which have smooth borders
• Mazabraud syndrome: FD + intramuscular myxomas (rare association)
• Malignant transformation: rare (<1% in monostotic; up to 4% in polyostotic; higher post-radiation) — most commonly to osteosarcoma; rapid growth or sudden pain warrants biopsy

• Medullary bone replaced by cellular fibrous stroma containing irregularly shaped woven bone trabeculae without osteoblastic rimming
• Classic histological description: "Chinese characters" or "alphabet soup" pattern — curved, irregularly shaped woven bone trabeculae within cellular fibrous stroma; absence of osteoblastic rimming is key
• Cartilage islands: present in some lesions (fibrocartilaginous dysplasia variant) — do not indicate malignancy
• Affected bone: cortex thinned but usually intact; medullary cavity expanded; bone structurally weak and deformable
• The abnormal fibrous tissue cannot mineralise normally — this is why the bone remains structurally weak throughout life and deforms under load

• Ground-glass matrix: classic radiological appearance — homogeneous, hazy density of affected medullary bone resembling frosted glass; reflects the poorly mineralised fibrous tissue replacing normal bone
• Well-defined lesion with narrow zone of transition; endosteal scalloping; cortex thinned but generally intact; no periosteal reaction (unless fracture)
• Expansile lesion with cortical thinning — characteristic "rind" of thinned cortex around ground-glass interior
• Shepherd crook deformity: characteristic varus deformity of the proximal femur — repeated microfractures and plastic deformity of the femoral neck cause progressive varus, eventually producing a hook-shaped proximal femur resembling a shepherd crook; almost pathognomonic of fibrous dysplasia involving the proximal femur
• Bone scan: intense uptake in affected areas — useful for mapping extent of polyostotic disease
• MRI: variable signal (low T1; variable T2 depending on proportion of fibrous tissue vs cartilage vs cystic change); useful for assessing cortical integrity and soft tissue involvement

• Wide spectrum from asymptomatic (incidental finding) to severe deformity with recurrent fractures and chronic pain
• Proximal femur: most clinically significant location — progressive varus deformity (shepherd crook), limb length discrepancy, pathological fractures of the femoral neck; Trendelenburg gait from coxa vara
• Facial involvement: facial asymmetry, proptosis, visual compromise, hearing loss from skull base/orbital involvement; lion face (leontiasis ossea) in severe polyostotic disease
• Biochemistry: usually normal; elevated alkaline phosphatase in extensive polyostotic disease; phosphate wasting in some — FGF23 elevated (similar mechanism to XLH)
• McCune-Albright: girls present with vaginal bleeding and breast development before age 8 years (precocious puberty); assess with pelvic ultrasound and endocrine evaluation
• Worsening during pregnancy: FD lesions may enlarge and become more painful during pregnancy — oestrogen-responsive pathology

• Observation: asymptomatic monostotic disease with no deformity or fracture risk — observe with serial radiographs; many lesions stabilise at skeletal maturity
• Bisphosphonates (pamidronate IV): reduce pain and bone turnover in FD; some evidence of radiological improvement; do NOT cure FD but can reduce fracture pain and ALP; most evidence for IV pamidronate — oral bisphosphonates less well studied in FD
• Pain management: NSAIDs; bisphosphonates; neuropathic agents for bone pain; opioids for acute fracture pain
• Endocrine management in McCune-Albright: aromatase inhibitors or testolactone for precocious puberty; endocrinology co-management essential
• Activity modification: avoid high-impact activities that stress weakened bones; physiotherapy to optimise gait mechanics and muscle strength

Surgical intervention is indicated for pathological fractures, impending fractures, progressive deformity, and painful lesions failing conservative management. The fundamental principle of surgical management differs from standard fracture fixation.

• Key surgical principle: FD bone does not remodel normally — bone graft placed into FD lesions is resorbed and replaced by abnormal fibrous tissue; autograft and allograft behave identically poorly in FD cavities; cortical allograft slightly better than cancellous
• PMMA cement preferred over bone graft in FD — cement is not remodelled and provides durable fill; however, cement should be combined with internal fixation for structural support
• Proximal femur FD (shepherd crook deformity):

• Intramedullary nailing is preferred over plating for FD — spanning the entire lesion with an IM nail provides superior load-sharing and reduces fracture risk at stress risers beyond the implant
• Curettage alone without fixation: only for small contained lesions in non-weight-bearing bones; not sufficient for weight-bearing long bones
• Growing children: use flexible nails or fixation that can be revised; account for ongoing growth

• Avoid radiation: post-radiation sarcomatous transformation is a well-documented risk in FD — radiation should never be used; treat with surgery and medical management only
• Malignant transformation in FD: sudden increase in pain or rapid radiological change in a known FD lesion = biopsy urgently; osteosarcoma is most common secondary malignancy; fibrosarcoma and undifferentiated pleomorphic sarcoma also described; overall rate <1% monostotic, higher polyostotic
• Phosphate wasting in FD: FGF23 produced by abnormal stromal cells → renal phosphate wasting → secondary rickets-like picture; in extensive polyostotic disease this can be clinically significant; monitor renal tubular phosphate handling; treat with phosphate supplementation ± calcitriol as in XLH
• Bisphosphonate use caution: bisphosphonates inhibit osteoclast activity — theoretically beneficial but some evidence they harden FD bone and make surgical curettage more technically difficult; plan surgery before or early in bisphosphonate course if surgical intervention anticipated
• Craniofacial FD: managed by craniofacial surgeons; surgical debulking for cosmesis or functional compromise (visual loss, airway); timing controversial — stabilisation at skeletal maturity preferred before surgery as lesions may grow until then; optic canal decompression is urgent if visual acuity threatened

• GNAS activating mutation (Gs-alpha); somatic, post-zygotic; mosaic distribution; not hereditary
• Ground-glass matrix on X-ray = classic fibrous dysplasia appearance
• Shepherd crook deformity: varus proximal femur from repeated microfractures; almost pathognomonic of proximal femoral FD
• McCune-Albright: polyostotic FD + café-au-lait (jagged borders) + endocrine hyperfunction (precocious puberty)
• Chinese characters histology: curved woven bone trabeculae WITHOUT osteoblastic rimming — key distinguishing histological feature
• Bone graft does NOT work in FD — resorbed and replaced by fibrous tissue; PMMA cement preferred
• IM nail preferred over plate — spans lesion, reduces stress riser fracture risk
• Shepherd crook correction: valgus osteotomy + long cephalomedullary nail spanning whole lesion
• Radiation contraindicated — risk of post-radiation sarcoma
• Malignant transformation <1% monostotic; sudden pain change = urgent biopsy; osteosarcoma most common