Function Transfer forces from muscle to bone to produce joint motion Tendons orient themselves along stress Types Paratenon covered tendons e.g., patellar, achilles tendons have rich vascular supply and thus heal better often injured due to trauma and most often fail at the musculotendinous junction tendon-bone junction Sheathed tendons e.g., hand flexor tendons less vascularized and have avascular areas that receive nutrition by diffusion often injured due to laceration and at risk for adhesions Anatomy Composition groups of collagen bundles (fascicles) separated by endotenon and surrounded by epitenon composed of water tendons primarily composed of water collagen Type I collagen makes up 85% of dry weight of tendons Type III collagen make up 0-5% of dry weight of tendons proteoglycans make up 0-5% of dry weight of tendons decorin is the most predominant proteoglycan in tendon regulates collagen fiber diameter (length of 300nm, diameter of 1.5nm) forms cross-links between collagen fibers and transfers loads between collagen fibers aggrecan is proteoglycan found in areas of tendon compression Structure has a highly ordered hierarchical structure microfibrils insert into bone via 4 transitional tissues of increasing modulus tendon type I and III collagen, elastin, proteoglycans, tendon fibroblasts uncalcified fibrocartilage aggrecan, types I, II and III collagen, fibrochondrocytes tidemark - straight, basophilic line separating uncalcified and calcified fibrocartilage, a mechanical boundary between soft-hard tissue calcified fibrocartilage (separated from fibrocartilage by tidemark) type II collagen, aggrecan, types I and X collagen, fibrochondrocytes irregular boundary, with interlocking of calcified fibrocartilage zone with bone bone osteocytes, osteoclasts, osteoblasts, type I collagen, apatite Blood supply the musculotendinous junction the osseotendinous junction vessels from various surrounding connective tissue (such as the paratenon, mesotenon and vincula) FDS and FDP tendons have 2 vincula each (vincula longa and vincula brevia) Cell biology fibroblasts are predominant cell type spindle shaped and arranged in parallel rows in direction of muscle loading produce mostly type I collagen (85% of dry weight of tendons) produce small amount of type III collagen (5% of dry weight) responsible for healing process Fibrous vs. Fibrocartilaginous Enthesis Fibrous Enthesis (Indirect Attachment) Fibrocartilaginous Enthesis (Direct Attachment) Attachment Metaphysis and diaphysis of long bones Epiphysis and apophysis Composition Perforating mineralized collagen fibers 4 distinct zones (tendon, fibrocartilage, calcified fibrocartilage, and bone) Angle of Insertion Insertion angle changes slightly during motion Insertion angle changes greatly during motion (thus prone to overuse injury) Example Deltoid-humerus attachment, adductor magnus-linea aspera attachment, pronator teres attachment Rotator cuff, Achilles tendon Material Properties Characteristics tendons contain more collagen and are less viscoelastic than ligaments tendon enthesis contains free nerve endings responsible for nociception viscoelastic behavior with nonlinear elasticity the rate at which tendon sees force can influence the mechanical property biomechanical effects exercise has positive effect immobilization has detrimental effect age dependent increase in strength from birth to maturity decrease in strength after maturity laser/heat causes tendons to shrink vary with exposure to hydration, temperature, pH tendons should be tested under physiologic relevant conditions Advantages strong in tension (can withstand 5-10% as opposed to 1-4% in bone) Disadvantages buckle in compression demonstrate creep and stress relaxation Load-elongation or stress-strain curve toe region initial nonlinear segment of curve during low loads due to tendons being "crimped" linear region intermediate loads failure region high loads Tendon Healing Stages of tendon healing Stages of Soft Tissue Healing (including tendons) Hemostasis Platelets initiate coagulation cascade Fibrin clot and fibronectin interaction leading to chemotaxis to stabilize torn tendon edges 5-15 minutes Inflammation Fibroblasts produce type III collagen Macrophages help initiate healing and remodeling 1-7 days Proliferation Tissue modeling via large amounts of disorganized collagen and angiogenesis 7-21 days Remodeling Tissue remodeling replacing type III collagen to type I collagen up to 18 months Tendon Surgical Repair Strength following repair tendon repairs are weakest at 7-10 days most of strength by 21-28 days maximum strength at 6 months final strength only reaches 2/3 of normal even years after repair Early mobilization allows earlier ROM but decreased tendon repair strength beneficial for flexor tendon healing to prevent adhesion formation