Anatomy
Fascicles of long, spiraling bundles
- tenocytes & Type I collagen
- synovial cells & fibroblasts present
Endotenon
- surrounds the individual collagen bundles
Epitenon
- fine fibrous outer layer, highly cellular, continuous with endotenon
- contains most of the blood vessels & capillaries
Paratenon
- thin visceral layer of adventitia on tendon
- provides nutrition & allows gliding
Synovial Sheaths
- in distal palm & fingers, visceral synovial layer enclosing FDS/FDP
- parietal layer continuous with the pulleys
- tendons attached via long & short vinculae
Pulleys
Thickenings of the synovial sheath
- 5 strong annular pulleys interposed by 3 collapsible cruciate pulleys
- allow the annular pulleys to approximate in flexion
A2 & A4
- fibro-osseous annular pulleys
- arise from periosteum of the phalanx
- maintain short moment arm of tendon from joint, greatest joint rotation for least excursion
- most important
A1, A3, A5
- arise from the volar plates
- MCPJ, PIPJ & DIPJ respectively
Palmar Aponeurosis Pulley
- important additional pulley
- transverse fibres of palmar fascia
Thumb Pulleys
- A1 (MCPJ) and A2 (IPJ)
- Oblique pulley in between and is most important
- can be excised if A1 intact
Flexor Tendons
Excursion can exceed 8cm
- in pulley area flexor tendons have segments that are avascular
Actions
FDS
- arises from single muscle belly
- volar aspect of humerus, radius and ulna
- separates into 4 tendons in forearm
- IF and LF deep, RF and MF superficial in carpal tunnel
- LF may be absent (20%)
- bifurcates at level A1 pulley
- 2 slips rotate around and insert volar aspect base of P2 and radial / ulna sides
Action
- has independent action
- FDS & interossei combine for forceful flexion
- 200N achieved in power grip
FDP
- has common muscle origin
- arises volar aspect ulna and interosseous membrane
- deep to FDS
- several digits have simultaneous action
- acts as primary digital flexor
Lumbricals
- arise from FDP
- lateral 2 (ulna n) bipennate, medial 2 arise from 1 tendon only (median n)
- insert on radial side of extensor expansion
- flex MCPJ and extend IPJ's
Vascular Supply
Blood vessels
1. Longitudinal vessels enter tendons in palm
- Vessels enter at proximal synovial fold in distal palm
2. Vessels enter at osseous insertions
3. Segmental branches of digital arteries enter via long & short vinculae
- VBP vinculae brevis profundus
- VLP vinculae longus profundus
- VBS vinculae brevis superficialis
- VLS vinculae longus superficialis
Flexor tendons have highest vascularity dorsally
Synovial Fluid Diffusion
May function better than vascular perfusion
- composition similar to joint fluid
- imbibition process
- fluid is pumped into interstices of tendon through ridges oriented at 90° to each other during flexion and extension
- synovial sheath is critical to this process
- lacerations disrupt this mechanism
Avascular segments
1. FDS & FDP have avascular segments over proximal phalanx under A2
2. FDP has 2nd avascular segment over middle phalanx under A4
Tendon Biochemical Composition
Composition
- Type I collagen 95%
- Type III & V collagen 5%
Dense, parallel collagen fibres
- Highest tensile strength of all soft tissues
- Collagen in triple helix of tropocollagen molecules
Age and immobilization
- increases collagen content
- loss of water content, glycosaminoglycan concentration & strength
Exercise training
- increases collagen fibril size
- increases strength & stiffness
Tendon Healing
Both Intrinsic & Extrinsic factors
- extrinsic - fibroblasts and inflammatory cells from periphery
- intrinsic - fibroblasts and inflammatory cells from epitenon
Aim is to optimize intrinsic healing and minimize extrinsic healing which may lead to development of adhesions
3 Phases
- inflammatory
- fibroblastic
- remodelling
Inflammatory Phase Day 1-4
Clot fills defect
- Epitenon cells migrate into & bridge the gap
- Peritendinous cells proliferate & migrate into laceration site
Fibroblastic Phase Day 5-28
Collagen secretion begins by day 5
- fibres formed in random fashion
- Fibroblasts become the predominant cell type
- Synovium is reconstituted by day 21
- Vascularisation increases with penetration of avascular zones by new blood vessels
- Increased strength by 2 - 3 weeks
- Collagen content increases for first 4 weeks
- Collagen reorientation complete by day 28
Remodelling Phase Day 28-112
By day 28 fibroblasts longitudinally oriented
- progressive remodeling & realignment of collagen fibrils
- By 6 weeks gap is completely filled
- By 8 weeks collagen is mature & realigned
By 4 months
- maturation complete
- fibroblasts now quiescent tenocytes
- Full tensile strength only reached after physiologic loading
Adhesions
Dense adhesive scar
- results from ingrowth of fibroblasts from the digital sheath & epitenon proliferation
More severe
- immobilized tendons
- increased severity of synovial sheath injury /crush
- gaps > 3mm