Metastatic Bone Disease

Metastatic bone disease is far more common than any primary bone malignancy. The skeleton is the third most common site of metastases (after lung and liver). Skeletal metastases cause significant morbidity — pain, pathological fractures, spinal cord compression, and hypercalcaemia — and represent a major orthopaedic surgical burden. The orthopaedic surgeon must understand the biology, imaging, and surgical decision-making framework for metastatic bone disease, including the Mirels scoring system for impending fracture, the indications for prophylactic fixation, and the principles of surgical reconstruction.

Memory aid for the five most common primaries causing bone metastases: B-L-L-T-K (Breast, Lung, Lymphoma, Thyroid, Kidney) — `BLT with Ketchup`; or `Lead Kettle Brings Trouble Prematurely` (Lung, Kidney, Breast, Thyroid, Prostate)

The Mirels scoring system (1989) provides a quantitative framework for predicting pathological fracture risk in long bone metastases and guiding the decision for prophylactic surgical fixation.

• Advantages of prophylactic fixation over fracture treatment: lower surgical complexity; better outcomes; shorter hospital stay; faster rehabilitation; better quality of life; prophylactic fixation before fracture has significantly better functional results than fixation after fracture in metastatic disease; the decision to proceed with prophylactic fixation also requires consideration of estimated survival (surgery is justified if expected survival is >4–6 weeks — enough time to recover from surgery and benefit from improved mobility)

• General principle — `protect the whole bone`: when surgically stabilising a metastatic lesion in a long bone, the fixation device must protect the entire bone from other potential lesions — both existing and future; a short plate or nail that does not span the entire bone risks a second fracture through an unprotected region; for femoral metastases — a long cephalomedullary nail from the trochanter to the distal femur is preferred over a short nail; for humeral metastases — an anterograde intramedullary nail spanning the full humerus is preferred over a plate that only addresses the immediate fracture zone
• Intramedullary nail vs plate vs endoprosthesis: IM nail — preferred for diaphyseal and peritrochanteric long bone metastases (load-sharing device; spans the whole bone; applicable to impending and established fractures); plate — appropriate for metaphyseal lesions not amenable to nailing, or following curettage of accessible lesions; endoprosthesis (arthroplasty) — preferred when the femoral head/neck is destroyed (arthroplasty unavoidable) or for periacetabular lesions (cup +/- cage); for proximal femoral metastases involving the head and neck, hemiarthroplasty or total hip arthroplasty with long cemented stem (bypasses any femoral shaft lesions) is preferred over nailing
• Cement augmentation: injectable bone cement (PMMA) is used to fill the defect after curettage of the metastatic lesion and to augment the internal fixation construct; cement increases the rigidity and strength of the reconstruction and reduces the risk of hardware failure through weakened bone; cement is routinely used in arthroplasty for metastatic disease (cemented stems bypass deficient proximal femoral bone and provide immediate stability)
• Pre-operative embolisation for hypervascular metastases: renal cell carcinoma and thyroid metastases are highly vascular; pre-operative arterial embolisation (within 24–48 hours before surgery — within 48 hours to prevent revascularisation) significantly reduces intraoperative blood loss; failure to embolise a RCC bone met before surgery can result in life-threatening haemorrhage; embolisation of vertebral feeding arteries before corpectomy is particularly important
• Role of radiotherapy in bone metastases: RT is the primary treatment for pain control in metastatic bone disease; palliative RT (8 Gy single fraction or 30 Gy in 10 fractions) provides pain relief in approximately 60–70% of patients; post-operative RT (typically 2–4 weeks after wound healing) is given after fixation of all metastatic lesions to provide local tumour control and reduce the risk of hardware failure from continued tumour growth; RT should not delay surgery if surgical fixation is needed

• A patient presenting with a bone lesion possibly representing metastatic disease but no known primary requires systematic investigation; the most common unknown primaries presenting with bone metastases are lung (most common), kidney, and breast
• Workup for unknown primary: full history (weight loss, cough, haematuria, breast lump, smoking, previous malignancy); blood tests — FBC, metabolic panel, LFTs, ALP, PSA (men), serum protein electrophoresis, serum calcium, LDH, TFTs; tumour markers — CEA (colorectal/lung), CA-125 (ovarian), CA 19-9 (pancreatic), AFP (hepatocellular), β-hCG (germ cell); CT chest/abdomen/pelvis with contrast (identifies the primary in 50–70% of unknown primaries); PET-CT (higher sensitivity); mammography (women); bone marrow biopsy if haematological malignancy suspected; BIOPSY the bone lesion if the primary remains unknown after imaging workup — guided by biopsy principles (biopsy along the planned surgical approach, single-portal technique, send for histology + immunohistochemistry)
• Histological clues on biopsy: clear cells = RCC or clear cell sarcoma; signet ring cells = gastric/lobular breast; columnar cells with mucin = colorectal; organoid architecture = carcinoid/neuroendocrine; osteoclast-like giant cells = giant cell tumour (not always metastatic); immunohistochemistry panel tailored to histological appearance

• Metastatic spinal disease requires assessment of three factors: (1) spinal instability; (2) neurological involvement; (3) systemic disease burden
• Spinal Instability Neoplastic Score (SINS): a validated scoring system for assessing spinal instability in metastatic disease; six parameters: junction location (occipitocervical, cervicothoracic, thoracolumbar, lumbosacral junctions — highest instability scores), pain character (mechanical pain on movement = instability), lesion type (lytic > mixed > blastic), radiographic spinal alignment (kyphosis/scoliosis), vertebral body involvement (>50% body involvement), posterior element involvement; Score 0–6 = stable (non-surgical); 7–12 = potentially unstable (multidisciplinary review); 13–18 = unstable (surgical evaluation recommended)
• ESCC (Epidural Spinal Cord Compression) grading: Bilsky scale grade 0 (bone only) → grade 1 (epidural involvement, no cord deformity) → grade 2 (cord deformed, not compressed) → grade 3 (cord maximally compressed); grades 2–3 with neurological deficit = surgical decompression indicated
• Separation surgery + stereotactic body radiotherapy (SBRT): modern concept for spinal metastases; rather than attempting radical tumour excision (high morbidity), a limited decompression and stabilisation is performed (`separation surgery` — creating a 2–3 mm gap between the tumour and the spinal cord); SBRT (high-dose, precisely targeted radiotherapy) is then delivered post-operatively to ablate the residual tumour; this combined approach achieves excellent local tumour control with lower surgical morbidity than radical excision; gaining widespread adoption at specialist centres

• Most common bone met primaries (BLT-KP): Breast (mixed), Lung (lytic, poor prognosis), Thyroid (lytic, hypervascular), Kidney/RCC (lytic, hypervascular — embolise pre-op), Prostate (OSTEOBLASTIC — hot bone scan, elevated ALP + PSA)
• Mirels score ≥9: prophylactic fixation; score 8: borderline; ≤7: RT; maximum 4 points per variable (site 3 = peritrochanteric; pain 3 = functional; lytic 3; >2/3 cortex 3)
• Protect the whole bone: nail the full length of the femur/humerus; short nail or plate = leaves distal bone unprotected against developing lesions; long cephalomedullary nail for proximal femur mets; anterograde nail for humerus
• RCC and thyroid mets: HYPERVASCULAR; pre-operative embolisation within 24–48 hours of surgery; failure to embolise = life-threatening haemorrhage; embolise vertebral feeders before spinal surgery
• Post-operative RT: given after wound healing (2–4 weeks) for all metastatic fixations; provides local tumour control; reduces hardware failure from progressive tumour; does not replace surgery; palliation not cure
• SINS score: 0–6 stable; 7–12 potentially unstable; 13–18 unstable → surgical evaluation; junction locations (occipitocervical, CTJ, TLJ, lumbosacral) = highest instability scores
• Separation surgery + SBRT: limited decompression creating a 2–3 mm gap between tumour and cord; followed by high-dose SBRT; excellent local control with lower morbidity than radical excision; modern standard of care at specialist centres
• Prophylactic fixation vs fracture surgery: always better outcomes before fracture (less complex, faster recovery, better function); justify surgery if expected survival >4–6 weeks
• Unknown primary: CT CAP + PET-CT finds primary in 50–70%; PSA (men), mammography (women), SPEP (myeloma), AFP (HCC), β-hCG (germ cell); biopsy along planned surgical approach if still unknown
• Vertebroplasty/kyphoplasty: painful vertebral compression fractures without cord compromise; PMMA cement; rapid pain relief; kyphoplasty restores height with lower cement leak risk; contraindicated if posterior cortex destroyed