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A torn Achilles tendon heals slowly over months, but a fractured bone heals in weeks. Tendons are dense regular connective tissue — mostly collagen with few cells and poor blood supply, so repair is slow. Bone is living, vascular tissue with osteoblasts and a rich blood supply, so fractures heal in 6–8 weeks. Cartilage heals worst of all — no blood vessels at all — which is why knee cartilage injuries often need surgery.