Chromosomal and Genetic Disorders in Orthopaedics

A Comprehensive Essay

Introduction

1. Classification and Overview

A. Chromosomal Disorders

• Result from numerical abnormalities (aneuploidy) or structural rearrangements (deletions, duplications, translocations, inversions) of whole chromosomes
• Examples with orthopaedic relevance: Down syndrome (Trisomy 21), Turner syndrome (45,X), Klinefelter syndrome (47,XXY)

B. Single-Gene (Mendelian) Disorders

• Caused by pathogenic variants in a single gene; follow autosomal dominant (AD), autosomal recessive (AR), or X-linked inheritance patterns
• Include the skeletal dysplasias (osteochondrodysplasias) such as achondroplasia, multiple epiphyseal dysplasia, and Marfan syndrome

C. Connective Tissue Disorders with Genetic Basis

• Affect the structural proteins of bone and connective tissue: collagen, fibrillin, elastin
• Examples: Osteogenesis Imperfecta (OI), Ehlers-Danlos Syndrome, Marfan Syndrome

D. Metabolic and Storage Disorders with Skeletal Involvement

• Enzyme defects that lead to accumulation of metabolites within bone and soft tissue
• Examples: Mucopolysaccharidoses (MPS), Gaucher's disease, Homocystinuria

2. Chromosomal Disorders

Down Syndrome (Trisomy 21)

• Most common autosomal chromosomal disorder; incidence ~1:700 live births
• 95% due to meiotic non-disjunction producing trisomy 21; 4% due to Robertsonian translocation; 1% mosaic
• Advanced maternal age is the strongest risk factor

• Atlantoaxial instability (AAI): Present in 10–30% of patients; caused by ligamentous laxity of the transverse ligament of C1; atlantodental interval (ADI) >5 mm on radiograph is diagnostic; can progress to myelopathy
  • Screening lateral cervical radiographs (flexion, extension, neutral) are mandatory before contact sports and surgical anaesthesia
  • ADI >10 mm or neurological symptoms mandate surgical stabilisation (C1-C2 posterior fusion)
• Pes planus (flat feet): Universal due to generalised ligamentous laxity; usually asymptomatic; orthotics for symptomatic cases
• Hip instability and dislocation: Acetabular dysplasia and femoral head abnormalities may develop; requires surveillance
• Scoliosis and kyphosis: Occur in up to 50% of patients; typically mild but can require bracing or fusion in severe cases
• Patellofemoral instability: Recurrent patellar dislocation due to hyperlaxity; surgical realignment may be needed
• Metatarsus primus varus and hallux valgus: Common in older patients
• Joint hypermobility: Universal; predisposes to sprains and subluxations

• Mandatory pre-operative cervical spine evaluation
• Hypothyroidism (common in trisomy 21) affects bone density and wound healing
• Cardiac defects (AV canal, VSD) present in ~40%; require cardiac clearance

Turner Syndrome (45,X and Variants)

• Affects females; 45,X monosomy in 50%; remainder have mosaicism (45,X/46,XX) or structural X-chromosome anomalies (isochromosome Xq, ring chromosome)
• Incidence ~1:2,500 live female births

• Short stature: Nearly universal; final height typically 143–147 cm; due to haploinsufficiency of the SHOX gene (short stature homeobox gene) on the pseudoautosomal region of the X chromosome
• Cubitus valgus (increased carrying angle): A classic physical sign; caused by hypoplasia of the medial humeral condyle
• Madelung deformity: Shortening and bowing of the radius with dorsal subluxation of the distal ulna; due to SHOX haploinsufficiency causing premature fusion of the medial radial physis; leads to wrist pain and limited forearm rotation
• Scoliosis: Occurs in ~10%; may be accelerated by growth hormone therapy; regular monitoring required
• Genu valgum (knock knees): Common; usually managed conservatively
• Osteoporosis: Oestrogen deficiency (ovarian failure) leads to reduced bone mineral density; pathological fractures may occur; hormone replacement therapy (HRT) is essential
• Kyphosis: Thoracic kyphosis can develop, especially with scoliosis
• Lymphoedema of hands and feet: Present at birth in many; a classic clinical clue

• Growth hormone therapy improves final height
• HRT essential from age ~12 years to prevent osteoporosis and promote bone accrual
• Orthopaedic intervention for Madelung deformity if symptomatic (physiolysis, corrective osteotomy)
• Dual-energy X-ray absorptiometry (DEXA) scanning to monitor bone density

Klinefelter Syndrome (47,XXY)

• Most common sex chromosome abnormality in males; incidence 1:500–1:1,000 live male births
• Non-disjunction in either parent; 47,XXY in 80%; mosaic forms and higher-grade aneuploidies (48,XXXY; 49,XXXXY) also occur — these have more severe manifestations

• Tall stature with disproportionately long lower limbs: Decreased testosterone leads to delayed growth plate closure; increased lower segment/total height ratio
• Osteoporosis: Due to hypogonadism; fracture risk significantly elevated, particularly vertebral compression fractures
• Scoliosis and kyphosis: Occur at higher frequency than in the general population; core muscular weakness contributes
• Joint hypermobility: Ligamentous laxity from testosterone deficiency
• Pes planus and genu valgum: Common orthopaedic findings
• Muscle weakness and hypotonia: Generalised; contributes to postural deformity and functional limitations

3. Skeletal Dysplasias (Osteochondrodysplasias)

Achondroplasia

• Most common non-lethal skeletal dysplasia; incidence ~1:25,000 live births
• Autosomal dominant mutation in the FGFR3 gene (fibroblast growth factor receptor 3), chromosome 4p16.3
• Gain-of-function mutation (Gly380Arg in >97% of cases); 80% arise as de novo mutations in advanced paternal age
• FGFR3 normally inhibits chondrocyte proliferation; the activating mutation causes excessive inhibition → impaired endochondral ossification

• Rhizomelic dwarfism: Shortening most severe in the proximal limb segments (humerus and femur); mean adult height ~131 cm (men), ~124 cm (women)
• Macrocephaly with frontal bossing; midface hypoplasia; depressed nasal bridge
• Trident hand configuration
• Lumbar hyperlordosis (pathognomonic); thoracolumbar kyphosis in infancy (usually resolves with ambulation)
• Varus deformity of knees (genu varum)
• Foramen magnum stenosis: Critical orthopaedic/neurosurgical issue in infancy; can cause sleep apnoea, sudden infant death, and long tract signs; requires urgent decompression if symptomatic
• Spinal stenosis: Develops in adults due to shortened pedicles and thick laminae; most significant cause of disability in adult achondroplasia; symptoms include claudication, radiculopathy, and paraplegia
• Thoracolumbar kyphosis: Persistent gibbus in ~10%; spinal arthrodesis with instrumentation required if severe (especially if >30 degrees with neural compromise) — Miller's Review of Orthopaedics, 9th ed.

• Squared iliac wings with horizontal acetabular roofs ("tombstone" pelvis)
• Champagne glass pelvic inlet
• Narrowing of the interpedicular distance in the lumbar spine (caudal tapering — opposite to normal)
• Rhizomelic shortening of long bones with metaphyseal flaring

• Vosoritide (CNP analogue, FGFR3 antagonist) — approved for children with open growth plates to increase growth velocity
• Limb lengthening (controversial): Femoral and tibial osteotomies; psychological and social considerations important
• Foramen magnum decompression in symptomatic infants
• Lumbar laminectomy/decompression for spinal stenosis in adults
• Corrective osteotomy for severe genu varum

Multiple Epiphyseal Dysplasia (MED)

• Heterogeneous condition; AD forms due to mutations in COMP (cartilage oligomeric matrix protein), COL9A1/2/3, MATN3; AR form due to SLC26A2 mutation
• Disrupts normal epiphyseal ossification

• Short stature (mild); waddling gait; joint pain and stiffness from early childhood
• Bilateral, symmetric, irregular, fragmented epiphyses on radiograph — particularly hips, knees, and ankles
• Early-onset osteoarthritis of hips and knees — the principal source of adult morbidity
• Must be differentiated from Perthes disease (bilateral Perthes is often MED)
• Management: Activity modification; joint replacement when osteoarthritis develops; NSAID analgesia — Miller's Review of Orthopaedics, 9th ed.

Diastrophic Dysplasia

• AR; mutations in SLC26A2 (sulphate transporter gene), impairs sulphation of proteoglycans → defective cartilage matrix

• Severe short-limb dwarfism
• Hitchhiker thumb (characteristic abducted, proximally placed thumb)
• Severe clubfoot (resistant to standard casting)
• Progressive scoliosis (can be severe; requires early surgical fusion)
• Cervical spine kyphosis — can cause cord compression
• "Cauliflower ears" from recurrent haematomas

4. Connective Tissue Genetic Disorders

Osteogenesis Imperfecta (OI)

• A phenotypically and genetically heterogeneous generalised connective tissue disorder; incidence ~1:10,000–15,000 births — Harrison's Principles of Internal Medicine, 22nd ed.
• Types I–IV (most common) are caused by quantitative or structural defects in type I collagen encoded by COL1A1 (chromosome 17q) and COL1A2 (chromosome 7q); autosomal dominant inheritance
• Type I: Reduced quantity of structurally normal collagen; mildest form
• Type II: Lethal perinatal; in utero fractures of ribs and long bones
• Type III: Progressive deforming; moderately severe; most severe non-lethal form
• Type IV: Mild-to-moderate fragility; variable sclerae
• Types V–XXII involve mutations in genes affecting collagen post-translational modification, cross-linking, matrix mineralisation, and osteoblast differentiation — Harrison's, 22nd ed.

• Bone fragility: Hallmark; frequency of fractures varies from several childhood fractures (type I) to in utero/perinatal fractures (type II); fractures heal but deformity accumulates — Miller's Review, 9th ed.
• Blue sclerae: Classic in types I and II; due to thinness of sclerae allowing choroidal veins to show through
• Dentinogenesis imperfecta: Opalescent teeth that wear rapidly
• Hearing loss: Otosclerosis-like; mixed or sensorineural
• Basilar invagination: Upward migration of the dens into the foramen magnum in severe types; causes headache, cranial nerve palsies, hydrocephalus, and long-tract signs — Miller's Review, 9th ed.
• Ligamentous laxity and joint hypermobility
• Scoliosis and kyphosis: Progressive; can compromise pulmonary function
• Cardiac valve abnormalities (mitral valve prolapse, aortic regurgitation)
• Growth deficiency: Progressive, especially in types III and IV

• Bisphosphonates (intravenous pamidronate or zoledronic acid): Reduce fracture incidence by increasing bone density; first-line in moderate-to-severe OI — Miller's Review, 9th ed.
• Intramedullary rodding (telescoping rods — Fassier-Duval or Bailey-Dubow): Indicated for recurrent long bone fractures and progressive deformity; telescoping rods accommodate growth
• Corrective osteotomies for bowing deformity of femur and tibia
• Spinal surgery: Posterior spinal fusion for progressive scoliosis; posterior fossa decompression for basilar invagination
• Rehabilitation: Physiotherapy, hydrotherapy, assistive devices; avoid immobilisation as it accelerates bone loss

Marfan Syndrome

• AD; mutations in FBN1 gene (chromosome 15q21.1) encoding fibrillin-1, a glycoprotein critical for microfibrils in connective tissue
• TGF-β signalling dysregulation is a key downstream mechanism
• Incidence ~1:5,000; 25% de novo mutations

• Skeletal: Tall stature; dolichostenomelia (long limbs relative to trunk); arachnodactyly (long spider-like fingers); arm span > height; pectus excavatum or carinatum; scoliosis in up to 60% (often double major curves; surgery required when >45 degrees); dural ectasia (enlargement of dural sac, particularly lumbosacral) — a radiological hallmark causing low back pain and sciatica
• Protrusio acetabuli: Medial displacement of the femoral head into the pelvis; seen on radiograph; associated with groin pain and early arthritis; treated with total hip arthroplasty when symptomatic
• Pes planus and hindfoot valgus: Common
• Joint hypermobility and recurrent dislocations: Especially shoulder and patella
• Cardiovascular: Aortic root dilatation → aortic dissection (life-threatening); mitral valve prolapse — close cardiac surveillance is mandatory
• Ocular: Ectopia lentis (upward lens subluxation) in >50%; high myopia

• Scoliosis surveillance: Bracing generally ineffective; posterior spinal fusion when >45–50 degrees
• Dural ectasia: Usually conservatively managed; CT myelogram or MRI for diagnosis
• Protrusio acetabuli: Acetabular reinforcement ring + cemented cup at THA
• Avoid contact sports due to aortic dissection risk; β-blockers or losartan reduce aortic enlargement rate

Ehlers-Danlos Syndrome (EDS)

• A clinically and genetically heterogeneous group of >13 subtypes
• Most common: classical EDS (AD; COL5A1/COL5A2 mutations) and hypermobile EDS (hEDS; no confirmed gene yet)
• Kyphoscoliotic EDS (AR; PLOD1 or FKBP14 mutations): most severe orthopaedic subtype
• Vascular EDS (AD; COL3A1): risk of arterial rupture; least amenable to surgery

• Joint hypermobility: Universal; Beighton score used to quantify; leads to repeated sprains, subluxations, and dislocations
• Chronic joint pain and proprioceptive deficits
• Scoliosis: Especially in kyphoscoliotic type; can be progressive and severe; may require surgical fusion
• Skin fragility and poor wound healing: Critical surgical consideration; sutures hold poorly; staged closure and specialised techniques required
• Pes planus and joint deformity from ligamentous laxity
• Recurrent shoulder and patellar dislocations: Surgical stabilisation complex and often results suboptimal
• Cervical instability: Can mimic atlantoaxial instability

5. Metabolic and Storage Disorders Affecting Bone

Mucopolysaccharidoses (MPS)

• A group of lysosomal storage disorders caused by deficiencies of lysosomal enzymes that degrade glycosaminoglycans (GAGs); AR inheritance (except Hunter syndrome, MPS II — X-linked)
• Accumulation of GAGs (dermatan, heparan, keratan, chondroitin sulphate) in tissues including bone, cartilage, ligaments, and spinal cord

• Dysostosis multiplex: Radiological constellation including J-shaped sella turcica, paddle-shaped ribs, spatulate clavicles, hip dysplasia, genu valgum, and flared iliac wings
• Gibbus deformity (thoracolumbar kyphosis): Due to hypoplastic vertebral bodies; can cause spinal cord compression
• Atlantoaxial and craniovertebral instability: Critical; due to odontoid hypoplasia (especially in Morquio syndrome, MPS IV) and GAG deposition in the transverse ligament; may require C1-C2 or occipitocervical fusion
  • Morquio syndrome (MPS IV): Most severe orthopaedic phenotype; odontoid hypoplasia with ligamentous laxity → risk of catastrophic spinal cord injury with minor trauma; screening flexion-extension cervical MRI mandatory before surgery or general anaesthesia
• Carpal tunnel syndrome: GAG deposition in flexor retinaculum; surgical decompression beneficial
• Hip dysplasia: Coxa valga with femoral head coverage deficiency; may require osteotomy or arthroplasty
• Stiff joints: In contrast to conditions above, MPS causes restricted joint movement (not hypermobility) due to intra-articular GAG deposition
• Short stature: Variable by type; most severe in MPS IV and MPS IH (Hurler)

• Enzyme replacement therapy (ERT): Modifies systemic disease but limited CNS and bone penetrance
• Haematopoietic stem cell transplantation (HSCT): Effective in MPS IH if done early (<2 years); halts neurocognitive decline; does not reverse established skeletal deformity
• Orthopaedic surgery: Spinal fusion for instability; carpal tunnel release; hip reconstruction; knee/ankle corrective procedures
• Anaesthetic risk: Difficult airway, short neck, restricted mouth opening, cervical instability — senior anaesthetic involvement mandatory

Homocystinuria

• AR; deficiency of cystathionine β-synthase (CBS gene); leads to accumulation of homocysteine
• Homocysteine impairs collagen and elastin cross-linking

• Marfanoid habitus: Tall, slender, arachnodactyly, pectus deformity — but must be differentiated from Marfan syndrome
• Scoliosis and vertebral osteoporosis with biconcave (codfish) vertebral bodies and compression fractures
• Genu valgum and pes planus
• Ectopia lentis (downward subluxation — opposite direction to Marfan)
• Thromboembolic risk: Markedly elevated; surgery in homocystinuria patients requires aggressive DVT prophylaxis (peri-operative anticoagulation, heparin infusion); anaesthetic risk of intraoperative thrombosis is significant
• Joint stiffness (unlike Marfan hypermobility)

6. Other Genetic Conditions of Orthopaedic Importance

Neurofibromatosis Type 1 (NF-1)

• AD; NF1 gene (chromosome 17q11.2) encoding neurofibromin (tumour suppressor, RAS-GAP); incidence ~1:3,000
• 50% de novo mutations

• Congenital pseudarthrosis of the tibia (CPT): Most challenging; anterolateral bowing of the tibia with pathological fracture that fails to heal; NF-1 present in ~50% of CPT; treatment involves intramedullary rodding, bone grafting, BMP-2, free vascularised fibular graft, Ilizarov technique — often requires multiple procedures
• Scoliosis: Two types:
  • Dystrophic (short, sharp angular curves; rib pencilling; vertebral scalloping; dumbbell neural foraminal tumours): Progressive; requires early surgical fusion; anterior and posterior approach often needed
  • Non-dystrophic (resembles adolescent idiopathic scoliosis; better prognosis)
• Osteoporosis and subperiosteal neurofibromas causing bone erosion
• Plexiform neurofibromas adjacent to joints causing overgrowth syndromes
• Leg length discrepancy secondary to tibial pseudarthrosis

Osteopetrosis (Marble Bone Disease)

• Heterogeneous; AD (benign) and AR (malignant, severe) forms
• Most common AR form: loss-of-function mutations in TCIRG1 (vacuolar proton pump subunit) or CLCN7 (chloride channel); leads to failure of osteoclast function → dense, sclerotic but brittle bone

• Pathological fractures through sclerotic bone (chalk-stick fractures); bone is dense but fragile; fractures heal slowly with malunion risk
• Bone-in-bone appearance on radiograph; obliteration of medullary canal; Erlenmeyer flask deformity of distal femur
• Anaemia, thrombocytopenia, and hepatosplenomegaly: From obliterated medullary cavity (extramedullary haematopoiesis)
• Cranial nerve palsies: Optic nerve compression (blindness), facial nerve palsy, deafness from foraminal encroachment
• Osteomyelitis of the jaw (especially after dental extraction): Risk due to poor vascularity of sclerotic bone
• Increased fracture risk at time of intramedullary nailing (drilling through marble-hard bone); carbide-tipped drill bits required; care to avoid thermal necrosis

• Haematopoietic stem cell transplantation (HSCT): Only cure for malignant AR form; corrects osteoclast dysfunction by providing normal osteoclast precursors
• Orthopaedic management: Armstrong et al. (1999) describe outcomes of orthopaedic management — fracture fixation technically demanding; intramedullary devices preferred — Miller's Review of Orthopaedics, 9th ed.
• High-dose calcitriol (stimulates residual osteoclast activity) and interferon-γ in some protocols

Charcot-Marie-Tooth Disease (CMT)

• Most common hereditary motor and sensory neuropathy; incidence ~1:2,500
• CMT1A (most common, ~70%): AD; duplication of PMP22 gene (chromosome 17p11.2) → demyelination
• Multiple genetic subtypes exist (CMT1B, CMT2A, CMTX, etc.)

• Pes cavus (high-arched foot): Classic; CMT is the most common neurological cause of pes cavus — up to 67% of pes cavus cases have a neurological aetiology — Miller's Review, 9th ed.; caused by intrinsic muscle weakness with sparing of extrinsic muscles; Coleman block test assesses hindfoot flexibility
• Clawing of toes: From intrinsic minus foot
• Foot drop: From anterior compartment weakness
• Scoliosis: Occurs in up to 20–25%
• Wasting of intrinsic hand muscles and lower limb muscles (stocking-glove distribution)
• Spinal MRI: Mandatory to exclude intraspinal pathology in pes cavus — Miller's Review, 9th ed.

• Flexible hindfoot: Plantar fascia release + transfer of peroneus longus to brevis + metatarsal osteotomies (Jones procedure) + toe claw correction
• Rigid hindfoot: Triple arthrodesis
• Foot-drop: Ankle-foot orthosis (AFO) or posterior tibial tendon transfer
• Scoliosis: Bracing rarely effective due to progressive neurological deterioration; surgical fusion when >45 degrees

7. Peri-operative Considerations in Genetic/Chromosomal Disorders

8. Principles of Orthopaedic Management

General Principles Across All Conditions:

• Multidisciplinary care: Geneticist, orthopaedic surgeon, physiotherapist, neurosurgeon, cardiologist, and ophthalmologist as appropriate
• Genetic counselling: Offered to patients and families; especially important for AD conditions (50% transmission risk) and AR conditions (25% risk in siblings)
• Growth surveillance: Height, weight, and skeletal maturity monitoring using standardised growth charts for the specific condition
• Bone mineral density assessment: DEXA scanning in conditions with fracture risk (OI, Turner syndrome, Klinefelter syndrome, MPS)
• Imaging protocols: Standing full-spine radiographs for scoliosis surveillance; flexion-extension cervical radiographs in Down syndrome, Morquio, diastrophic dysplasia; MRI brain and spine in achondroplasia, NF-1
• Avoid unnecessary immobilisation: In OI, prolonged immobilisation accelerates disuse osteoporosis and increases fracture risk
• Pharmacological interventions:
  • Bisphosphonates in OI and osteoporosis secondary to chromosomal disorders
  • Vosoritide in achondroplasia (children with open growth plates)
  • ERT in MPS
  • HRT in Turner syndrome; testosterone replacement in Klinefelter syndrome
• Surgical timing: Corrective surgery generally delayed until skeletal maturity unless neurological compromise is present or deformity is rapidly progressive

Conclusion

• Miller's Review of Orthopaedics, 9th Edition (ISBN 9780443112140)
• Harrison's Principles of Internal Medicine, 22nd Edition (ISBN 9781265977061)
• Robbins & Kumar Basic Pathology (ISBN 9780323790185)
• Grainger & Allison's Diagnostic Radiology (ISBN 9780323760751)
• Thompson & Thompson Genetics and Genomics in Medicine, 9th Edition (ISBN 9780323547628)