Flexor Tendon Background



Fascicles of long, spiraling bundles

- tenocytes & Type I collagen

- synovial cells & fibroblasts present



- surrounds the individual collagen bundles



- fine fibrous outer layer, highly cellular, continuous with endotenon

- contains most of the blood vessels & capillaries



- 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




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  





- 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



- has independent action

- FDS & interossei combine for forceful flexion

- 200N achieved in power grip



- has common muscle origin

- arises volar aspect ulna and interosseous membrane

- deep to FDS

- several digits have simultaneous action

- acts as primary digital flexor 



- 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 



- 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




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