PROCEDURE

1. PRE-WORK (before lab) review the organization of the bony lower extremity from your atlas or on an articulated skeleton in the lab. 
    (Netter479) (Netter504)
    (Netter515) (Netter516) (Netter517)

a) On the distal end of the femur, the bone diverges into two articulating surfaces of the knee joint, the lateral and medial condyles of the femur.

b) The femoral condyles rest on plateaus of bone present on the tibia, the lateral and medial condyles of the tibia. This flat surface is commonly called the tibial plateau.

c) Lateral to the tibia is the smaller fibula, which has a head that articulates with the lateral side of the tibia. The tibia is the primary weight bearing bone of the leg, with the fibula stabilizing the ankle joint.

d) The tibia and fibula terminate at the ankle in a medial malleolus (tibia) and lateral malleolus (fibula) that articulate with the talus to form the ankle joint.

e) The most posterior (proximal) bones of the foot are the calcaneus and talus. These two bones are commonly classified as the 'hind-foot'

f) On the medial side of the calcaneus is a bony shelf, the sustentaculum tali. This forms a groove for the tendon of flexor hallucis longus to 'hook' around and into the foot. Posterior on the calcaneus is the calcaneal tuberosity, the attachment point for the calcaneal (Achilles) tendon.

The calcaneus can exhibit abnormal deposition of bone during life that forms a 'heel spur', most commonly where the plantar aponeurosis or the calcaneal tendon insert on the  bone. Chronic irritation at the spur can cause painful inflammation.

g) Directly anterior (distal) to the calcaneus and talus bones are the navicular and cuboid bones. The three cuneiform bones (medial, intermediate, and lateral) are just anterior to the cuboid bone. The navicular, cuboid and cuneiform bones are commonly classified as the 'mid-foot'.

g) The metatarsal bones (one for each digit) and the phalanges are collectively classified as the 'fore-foot'. There are two phalanges (proximal and distal) for the big toe and three phalanges (proximal, middle, and distal) for all the other toes.

Toes (and fingers) are numbered as well as having the common names. The big toe is number 1 and the little toe is number 5.

h) The bones of the foot are normally supported by ligaments that establish the arches of the foot (longitudinal and transverse arches).

Due to the arches, the primary weight bearing contact points of the foot are the calcaneus and the heads of the metatarsals.

i) The joints are named based on the bones located to each side of the individual joints. The main joints are:

Talocalcaneal (subtalar) joint - The articular surfaces between the talus and the calcaneus form the talocalcaneial joint, or more commonly known as the subtalar joint. This joint allows for inversion and eversion of the foot (playing no role in plantar flexion or dorsiflexion).

Talonavicular joint - The surfaces between the talus and the navicular form the talonavicular joints.

Calcaneocuboid joint - The joint surfaces between the calcaneus and the cuboid bone form the calcaneocuboid joint.

Transverse tarsal joint - Collectively, the talonavicular and calcaneocuboid joints form the transverse tarsal joint.

The subtalar, talonavicular, and calcaneocuboid joints form a complicated triple articulation allowing inversion and eversion. Such movements are essential to normal locomotion, especially on rough or uneven surfaces.

Tarsometatarsal joints - The bony skeleton of the foot has articulation points between the respective metatarsal bones and the cuneiform/cuboid bones. 

Metatarsophalangeal joints - These joints allow movements occur in several planes to effect flexion, extension, abduction, adduction, and circumduction of the toes.

Interphalangeal joints - These joints are articulations between each of the phalanges. Interphalangeal joints are hinge joints with movements of flexion and extension of the toes.

CLINICAL EXERCISE - Medial Malleolus Fixation

2. The leg and ankle are common locations for fractures, which can cause instability of the bony architecture of the ankle and need stabilization (fixation) with hardware.

In this clinical exercise, you will insert a bone screw into the tibia at the ankle to stabilize a malleolus fracture in an intervention. 

For the steps to take:
                                ------ click here------

PROCEDURE - Superficial Posterior leg

3. Turn the donor in the prone position and reflect the previously dissected skin away from the posterior thigh.

4. Extend the skin incision along the posterior side of the lower extremity from the popliteal down to the calcaneus (heel) along the midpoint of the posterior leg.
     (Figure505a) (Photo5113)

5. Start at the popliteal fossa begin reflecting skin medially and laterally to expose the crural fascia overlaying the posterior compartment of the leg.
    (Figure505c) (Netter474) (Photo5114)

As you reflect the skin and superficial fascia of the popliteal fossa and leg, look for the small saphenous vein and sural nerve connecting into the popliteal fossa from where they travel in the subcutaneous tissue of the posterior leg.  (Figure506) (Photo5007)

These most commonly enter/exit at the popliteal fossa, but either can penetrate the crural fascia partway down the leg and travel deep to the crural fascia to the popliteal fossa.

The sural nerve carries cutaneous sensory information is the 'nerve of choice' for nerve grafts. The nerve is long, easily accessible in the superficial fascia, and removal of the nerve has limited deficit (i.e. loss of sensation from the posterolateral distal leg and foot).

6. Reflect any remaining skin and superficial fascia from the posterior thigh and leg as far as the calcaneus (heel)
    (Figure506) (Photo5115)

Preserve a segment (~10-15cm) of the small saphenous vein and sural nerve if observed.

You may observe numerous additional small veins within the superficial fascia which are not individually named. These form the superficial venous system of the lower extremity and can be removed as needed.

These superficial veins are a common site for benign varicosities. Varicose veins is the term used for when a vein becomes twisted and enlarged. Risk increases with age and are most common in the superficial venous system in the leg due to gravity load on distal inferior veins.

While the term 'varicose veins' applies to any vein in the body undergoing this process, the term is almost synonymous in common vernacular with occurrences in the leg due to the high probability of varicose veins occurring there.

7. On the lateral side of the heel, make a tangential incision linking this midline leg incision to the previous incision along the lateral aspect of the foot.
    (Figure505d) (Photo5116)

8. On the medial side of the heel, make a tangential incision linking this midline leg incision to the previous hockey-stick incision at a point anterior to the malleolus.
    (Figure505b) (Photo5117)

9. Reflect skin from the calcaneus (heel) to over the medial and lateral malleoli.
    (Netter520) (Photo5118) (Photo5118a)

This fully exposes the calcaneal (Achilles') tendon and malleolus on each side of the ankle.

10. Using scissors make a longitudinal incision in the posterior surface of the crural fascia from the level of the popliteal fossa to the calcaneus following the line of the previous skin incision.
    (Netter474) (Netter507) (Netter514) (Photo5119)

The crural fascia is a dense fascia that fuses around the knee and is continuous with the fascia lata proximally. This fascia forms a sleeve surrounding the leg.

An extension of this fascia, the transverse intermuscular septum, divides the posterior leg into a superficial posterior and deep posterior compartment. This will be examined in following steps.

Additional septa form the anterior and lateral compartments of the leg. 

11. Starting at the popliteal fossa, identify the two heads of the gastrocnemius muscle and follow each head proximally to their attachments to the epicondyles of the femur.
    (Figure510) (Netter507) (Photo5120) (Photo5011)

The gastrocnemius muscle is the largest and most superficial muscle in the posterior superficial compartment of the leg.

The muscle spans both the knee and ankle joints and acts on both joints (knee flexion and ankle plantar flexion).

12. Follow the gastrocnemius muscle distally and observe that the two heads of the muscle come together and form a tendon which merges into the calcaneal tendon.
    (Figure510) (Netter507) (Photo5120)

The calcaneal tendon is commonly called the Achilles' tendon and inserts onto the calcaneal tuberosity (most posterior bone of the foot).

The calcaneal tendon is formed by the merging of the tendons from all the posterior superficial compartment muscles (i.e. the gastrocnemius, soleus, and plantaris tendons).

13. Remove any skin and subcutaneous tissues posterior to the calcaneal tendon so that you can observe the tendon connection to the calcaneal bone.
    (Netter507)

14. Use fingers in the proximal leg where the muscle belly of the gastrocnemius is present and slide your fingers under the gastrocnemius muscle to separate it from the underlying soleus muscle.
    (Netter508) (Photo5121) (Photo5121a)

The deeper soleus muscle converges into the shared calcaneal tendon. Thus, the separation between the gastrocnemius and soleus muscles is only present in the proximal portion of the leg. 

15. Examine the superficial surface of the soleus muscle where it forms a broad sheet of muscle deep to the gastrocnemius.
    (Figure510) (Netter508) (Photo5122)

The soleus muscle arises from both tibia and fibula and forms a tendinous arch between the bones at the lower border of popliteus.

The popliteal artery and tibial nerve pass deep to this tendinous arch as they course inferiorly.

Since the soleus muscle crosses only the ankle joint the muscle action is plantar flexion at the ankle.

16. Isolate the plantaris tendon running along the surface of the medial side of the soleus muscle, sandwiched between the soleus and gastrocnemius muscle bellies.
    (Figure510) (Netter508) (Photo5123) (Photo5123a)

The plantaris tendon is a long thin tendon that also joins into the shared calcaneal (Achilles) tendon.

The tendon can range from a several millimeter wide strip down to a narrow 1mm strand. In approximately 10% of individuals it may be entirely missing. If absent, that can occur unilateral or bilateral.

17. Follow the plantaris tendon proximally and look for the slender muscle belly of the plantaris muscle between the heads of the gastrocnemius muscle.
    (Figure510) (Netter508) (Photo5124)

The plantaris muscle originates from the lateral epicondyle, thus as you follow the plantaris tendon proximally it will curve towards the lateral side deep to the space between the heads of the gastrocnemius muscle in fat of the popliteal fossa.

Blunt dissecting from the superior side between the heads of the gastrocnemius muscle while another team member gives gentle tugs on the plantaris tendon will help isolate the muscle belly from surrounding fat of the popliteal fossa.

The functional significance of this muscle is debated. One hypothesis is that the muscle provides balance  proprioceptive feedback due to the unusually high density of proprioceptive receptor organs in the muscle and tendon.

However, given the muscle is absent in many individuals and removal has no measurable consequences, the proprioception role must be minimal. As such, the plantaris is commonly used when a long thin tendon is needed for grafting purposes (e.g. tendon reconstructions in of the hand).

PROCEDURE - Deep Posterior Leg

18. Push a finger or blunt probe under the calcaneal tendon to isolate it from underlying connective tissue and transect the tendon ~3 cm superior to its calcaneal attachment
    (Photo5125)

The location transected, about 3-4cm distal to the calcaneal bone, is the most common location for an Achilles tendon rupture.

This risk zone is due to the low vascular supply occurring in the 2-6cm region proximal to the insertion. This area has reduced perfusion with age resulting in an area prone to degeneration and rupture during sudden high-intensity stress without prior training (e.g. athletic "weekend warrior" activities).

19. Lift the calcaneal tendon and begin to reflect the gastrocnemius, plantaris tendon and soleus muscles as a unit.
    (Photo5126)

20. As your reflect the calcaneal tendon and associated muscles, progressively transect the soleus muscle attachments along the medial and lateral muscle edges.
    (Figure550) (Photo5127) (Photo5127a)

The soleus muscle has strong attachments along the tibia (medial side) from near the knee to mid-tibial shaft that is being transecting in this reflection.

The lateral side mostly has light connective tissue along the fibula shaft, although this may require scissor transection. The only direct soleus attachment to the fibula is proximal near the knee.

 21. Stop the transection close to the knee, leaving the most proximal attachment of the soleus muscle to the tibia and fibula intact.
    (Figure550) (Photo5128)

This results in reflection of the three superficial posterior compartment muscles attached at their origin (left and right epicondyles of the femur for the gastrocnemius muscle, proximal tibia/fibula for the soleus, and lateral epicondyle of the femur for the plantaris muscle).

Separating the soleus muscle from the medial edge of the tibia is the surgical approach used in a fasciotomy procedure to access the deep posterior compartment. This is performed as part of a multi-compartment fasciotomy (e.g. a four compartment fasciotomy) as a treatment for a major compartment syndrome, although through a simple skin incision not the major reflection we are performing of course!

22. Examine deep to the soleus muscle where there is a thin sheet of fascia spanning over the deep posterior compartment muscles.
    (Netter514) (Photo5128)

This separating layer of fascia is the transverse intermuscular septum separating the superficial and deep posterior compartments of the leg. Parts of this sheet can tear while reflecting the superficial muscles

23. Slit the thin transverse intermuscular septum and reflect or remove the fascia to expose the deep posterior muscles.
    (Netter514) (Photo5129)

24. Blunt dissect to observe the most lateral of the deep posterior compartment muscles, the flexor hallucis longus muscle arising from the mid to distal fibula shaft.
    (Figure551) (Figure513) (Netter509) (Photo5130)

The flexor hallucis longus is the most lateral muscle in the posterior deep compartment. Typically, it attaches to the distal half of the fibular and is rarely present in the more proximal leg.

The muscle originates from the fibula and inserts a tendon into the great toe distal phalanx (digit 1). 

25. Blunt dissect to observe the most medial of the deep posterior compartment muscles, the flexor digitorum longus muscle arising along the tibia.
    (Figure551) (Figure513) (Netter509) (Photo5130)

The flexor digitorum longus muscle is the most medial muscle in the posterior deep compartment. This muscle originates along most of the tibial shaft from near the knee to near the ankle

The muscle originates from the tibia and inserts a tendon that splits in the foot to attach to the distal phalanges of all four toes (digits 2-5).

26. Examine just medial to the flexor digitorum longus muscle for the tibial nerve.
    (Figure511) (Netter509) (Netter532) (Photo5131)

If you return to the popliteal fossa and give a gentle tug on the tibial nerve near the origin from the sciatic nerve, you may observe the movement of the nerve bundle more distal in the leg.

The tibial nerve passes from the popliteal fossa through a tendinous arch spanning the attachment points of the soleus muscle to the tibia and fibula (i.e. under the tendinous arch of the soleus muscle).

Rarely, this tendinous arch of the soleus muscle can compress the tibial nerve, causing an entrapment condition and resulting in distal neuropathy.

As the tibial nerve courses through the leg, the nerve gives off branches to each of the muscles of the posterior superficial and posterior compartments of the leg (i.e. all posterior leg muscles superficial and deep).

27. Examine adjacent/with the tibial nerve for rest of the posterior tibial neurovascular bundle containing posterior tibial artery, and posterior tibial vein.
    (Figure511) (Netter509) (Netter532) (Photo5131)

The popliteal vessels (artery and vein) give rise to the three primary branches traveling into the leg/foot (anterior tibial, posterior tibial, and fibular arteries).  We will not look for the branch points themselves as that would require disrupting the muscular attachments.

The posterior tibial artery and vein are branches from the popliteal vessels and travel into the posterior leg under the tendinous arch of the soleus muscle alongside the tibial nerve.

The popliteal vessels  also gives rise to the anterior tibial vessels, which pass to the anterior side of the interosseous membrane to supply the anterior compartment of the leg and dorsum of the foot.

28. Examine in the proximal leg lateral to the posterior tibial vasculature for branches of the fibular vessels coursing in the lateral side of the compartment.
    (Figure511) (Netter509) (Netter532) (Photo5132)

The posterior tibial vessels give rise to the fibular vessels which travel along the lateral side of the posterior deep compartment. The distal portion is commonly covered by part of the flexor hallucis longus muscle. The fibular vessels supply the lateral compartment via perforator branches.

In about 10% of individuals there are variant pathways for the fibular artery/vein. Thus, if you do not observe the vessels along the lateral side they may be taking an alternate pathway, though will we not look for them further in that case.

Technically, the fibular artery arises from a very short common trunk of the popliteal artery (called the tibial-fibular trunk), but for simplicity most sources just state the fibular artery arises from the posterior tibial artery.

29. Retract the posterior tibial neurovascular bundle and flexor digitorum longus medially to expose the centrally located tibialis anterior muscle.
    (Figure551) (Figure513) (Netter509) (Photo5133)

The muscle begins in that central position originating from the interosseous membrane, but as it progresses distally the tibialis posterior muscle passes deep to the flexor digitorum longus muscle.

Thus, you may need to slightly retract the flexor digitorum longus muscle and flexor hallucis longus to observe the tibialis posterior muscle.

The tibialis posterior sends a tendon to the plantar surface of the foot inserting primarily into the navicular bone, but also has tendon slips to other mid-foot bones.

30. Return to the flexor digitorum longus muscle and remove tissue superficial to the tendon in order to follow to where the tendon courses between the medial malleolus and calcaneus.
    (Figure514a) (Netter509) (Netter520) (Photo5134)

All posterior deep compartment structures pass posterior to the medial malleolus to enter the plantar foot. The tissue removed superficial to the flexor digitorum longus tendon is a part of the flexor retinaculum.

The flexor retinaculum is a strong fibrous band, extending from the medial (tibial) malleolus to the calcaneus. The flexor retinaculum has multiple deep partitions forming individual canals for passage of each tendon and the tibial neurovascular bundle (posterior tibial artery, vein, and tibial nerve).

31. Separate flexor retinaculum anterior to the flexor digitorum longus tendon to isolate the tibialis anterior tendon.
    (Figure514a) (Netter509) (Netter520) (Photo5135)

The tendon passes around the malleolus tight/close to the bone in a strong sheath that must be openned.

Observe that, although the tibialis posterior muscle is located in the middle in the leg, the tendon courses deep to the flexor digitorum longus tendon to take up the anterior position as it passes the medial malleolus.

The mnemonic "Tom, Dick, and A Very Nervous Harry" describes the anterior to posterior order of the tendinous and neurovascular structures as they pass around the medial malleolus.
    Tibialis posterior,
    flexor Digitorum longus,
    posterior tibial Artery and Vein, tibial Nerve,
    flexor Hallucis longus.

32. Separate flexor retinaculum posterior to the flexor digitorum longus tendon to isolate the posterior tibial neurovascular bundle as it passes under the flexor retinaculum.
    (Figure514a) (Netter509) (Netter520) (Photo5135)

This bundle consists of the tibial nerve, the posterior tibial artery, and posterior tibial vein. These pass through a facial canal under the flexor retinaclum, the tarsal tunnel, to enter the foot.

The posterior tibial pulse is most easily felt midway between the medial malleolus and the medial calcaneal tubercle.  But a pulse can also be detected slightly proximal or distal to that location.

The canal for the tibial nerve under the flexor retinaculum is called the tarsal tunnel. Any inflammation or compression of the tibial nerve as it passes through the tunnel can result in referred pain from the sole of the foot (i.e. tarsal tunnel syndrome, analogous to carpal tunnel syndrome in the hand).

33. Return to the flexor hallucis longus muscle and follow the muscle distally to where the tendon courses posterior to the tibial neurovascular bundle, separating flexor retinaculum tissue lateral to the tendon.
    (Figure514a) (Netter509) (Netter520) (Photo5136)

The tendon of flexor hallucis longus forms the 'harry' of the mnemonic of structures passing the medial malleolus and is the most posterior of those structures.

The flexor hallucis longus tendon is slightly deeper in the fascia than the other tendons (i.e. closer to the calcaneus bone).

The tendon of flexor hallucis longus lies in a groove inferior to a calcaneus shelf, the sustentaculum tali. The flexor digitorum longus and tibialis posterior lie superior to the sustentaculum tali.

You may be able to palpate the location on your foot and feel the flex of the tendons coursing superior or inferior to the shelf wiggling the respective toes.

PROCEDURE - Plantar Foot

34. Place two stacked wood blocks under the ankles so that the plantar surface of the foot is elevated to improve access.
    (Figure548)

35. Follow the calcaneal (Achilles') tendon to its attachment to the calcaneal tuberosity and removing skin and the calcaneal fat pad over the calcaneus (heel).
    (Figure552) (Photo5137) (Photo5137a)

The calcaneal fat pad is often thicker than initially appreciated, about 1cm thick.

Staying along the surface of the calcaneal bone from the calcaneal tuberosity as you make the turn to the plantar surface of the foot will assist in the tissue plane to follow.

The calcaneal fat pad is a mixed fibrous and fatty network of tissue providing shock absorption and protection to the bone of the calcaneus.

36. Transect the anterior end of the calcaneal fat pad and remove the entire pad as a piece.
    (Figure552) (Photo5137b)

The fat pad is very inflexible and it is hard to continue removing plantar skin with the pad connected.

The point to transect for pad removal is approximate. If you see the plantar aponeurosis (shiny white) or any muscle tissue (darker tone) that is a good point to stop as those are both anterior to the calcaneus.

37. Start removing skin of the plantar foot working from the calcaneus distally (anteriorly) along the plantar surface of the foot to expose the plantar aponeurosis.
    (Figure515a)

The plantar aponeurosis spans the longitudinal arch of the foot, attaching posteriorly to the calcaneus and anteriorly to proximal phalanges.

The skin is often tightly bonded to the underlying fascia and will commonly require careful scalpel cuts to separate skin from the underlying tissue.

The skin can be removed in multiple small sections if needed.

When removing skin from the sole of the foot take care as the subcutaneous tissue over the lateral and medial plantar surfaces can be quite thin.

38. Work distally (anteriorly) reflecting skin from the entire sole of the foot, extending as far laterally and medially as the bony margins of the foot to the base of the toes.
    (Figure516) (Netter523) (Photo5015)

On the lateral side of the foot this matches up with your previous lateral foot incision (i.e. from the side of the little toe to the side of the calcaneus). Extend the lateral side incision distally as needed to remove skin of the plantar surface.

On the medial side, this matches up with the previous hockey-stick incision made for the malleolus fixation clinical activity (i.e. from the malleolus along the side of the foot). Extend the medial side incision distally as needed to remove skin of the plantar surface.

39. Use scraping motions with a probe or scalpel handle (no blade) to clean subcutaneous fat from the plantar aponeurosis.
    (Figure516) (Netter523) (Photo5015)

The plantar aponeurosis consists of bands of longitudinal connective tissue extending out to each of the digits. At the distal end of the plantar aponeurosis, transverse fascicles of connective tissue support and stabilize the longitudinal bands.

Inflammation of these fibrous bands is called plantar fasciitis. Pain from this condition is usually most prominent along the medial side of the calcaneus as that side is under higher stress during locomotion or standing.

40. Work a probe under the plantar aponeurosis near the calcaneus and using the probe as a guide transect the aponeurosis close to the calcaneus.
    (Figure517) 

41. Retract the plantar aponeurosis and with blunt dissection detach it from the underlying tissue as far distally as the base of the toes.
    (Figure517a) 

The plantar aponeurosis frequently has attachments to the underlying first layer flexor digitorum brevis muscle. Thus, the aponeurosis may not come off as a continuous sheet or may result in elements of muscle fibers coming off with the aponeurosis.

The four 'layers of the foot' are located deep to the plantar aponeurosis.

Although these are layers separated by fascia forming compartments within the foot, they are not neat or well defined. Instead, the layers follow the curvature of the foot and bulge or depress at different points to cover the layer contents. Thus, do not expect to see actual neat layers or the fascia sheets.

While not visibly clear, foot layers do have some clinical significance. For example, infections can track within layers dictating where an abscess may spread easily. Also, while rare, compartment syndrome can occur in the foot resulting in pressure building up in compartments created by the layers.

42. Clean any remnants of the plantar aponeurosis and examine the flexor digitorum brevis muscle.
    (Figure518) (Netter524) (Photo5016)

The first layer of the foot contains three intrinsic muscles of the foot (intrinsic as the originate and insert entirely within the foot). These are the flexor digitorum brevis, the abductor hallucis, and abductor digiti minim muscles.

Flexor digitorum brevis muscle: This muscle is in the middle of the plantar surface of the foot attaching to the calcaneus. Individual thin tendons span from the muscle belly to each of the toes (i.e. digits 2-5).  

The two abductors primarily function to support the medial and lateral aspects of the bony arches of the foot during locomotion. We will not dissect these muscles.

Abductor hallucis muscle: This is the most medial muscle, spanning from the medial side of the calcaneus to its distal attachment on the medial side of the great toe (digit 1).

Abductor digiti minimi muscle: This is the most lateral muscle, spanning from the lateral side of the calcaneus to the lateral side of the little toe (digit 5). 

43. Transect the flexor digitorum brevis muscle close to the calcaneus and reflect the muscle distally to expose the second layer of the foot.
    (Figure519)

The muscle often does not reflect as a single unit and can separates into several bellies connected to their tendons and respective toes.

The second 'layer' of the foot contains two long, or extrinsic, tendons (extrinsic as the tendons originate from muscles in the leg) and two short intrinsic muscles (intrinsic as they originate and insert within the foot).

Between the flexor digitorum brevis muscle and the deeper structures are the plantar neurovascular bundles (branches of the posterior tibial vessels and tibial nerve into a lateral plantar and medial plantar neurovascular bundles, which further divide into digital branches to the toes).

We will not isolate these but you may see nerve branches while reflecting the flexor digitorum brevis muscle.

44. Examine near the toes for the flexor digitorum longus tendons to digits 2-5.
    (Figure522) (Netter525) (Photo5019)

The tendon of the flexor digitorum longus muscle split and sends separate tendon slips to each of the toes (digits 2-5), inserting into the distal phalanges.

45. Follow the flexor digitorum longus tendon proximal to where the separate tendons merge into one and observe the quadratus plantae muscle attaching to the flexor digitorum longus tendon.
    (Figure522) (Netter525) (Photo5019)

The quadratus plantae muscle is one of the second layer intrinsic muscles located directly deep to the flexor digitorum brevis muscle.

The quadratus plantae muscle arises from the calcaneus and inserts into the tendon of flexor digitorum longus. As the name implies, the quadratus plantae muscle is roughly quadrangular in shape.

Lumbricals - The other intrinsic muscle in layer two of the foot are the lumbricals. These are four small worm-like muscles located between the flexor digitorum longus tendons. Their role in toe movement is somewhat limited so we will not examine them.

46. Examine proximal to the big toe (digit 1) for the prominent flexor hallucis longus tendon.
    (Figure522) (Netter525) (Photo5019)

Follow the tendon of the flexor hallucis longus proximally to where it courses deep to the tendon of the flexor digitorum longus.

The flexor hallucis longus tendon is medial to the flexor digitorum longus tendons in the distal half of the foot, lining up with the position of the toes.

However, since the muscle themselves are located on the 'wrong' side of the leg, the tendons must crossover each other in the plantar foot to reach their targets (i.e. flexor hallucis longus muscle is on the fibula/lateral side of the leg and crosses to the medial big toe, while flexor digitorum longus muscle is on the tibia/medial side of the leg and crosses to the lateral small toes).

48. Examine the muscle tissue on each side of the flexor hallucis longus tendon to identify fibers of the flexor hallucis brevis muscle (a layer three muscle).
    (Figure522) (Netter525) (Photo5021)

Flexor hallucis brevis divides into medial and lateral heads, which lie on each side of the flexor hallucis longus tendon (i.e. the long tendon 'splits' the brevis musculature to each side of the long tendon).

The flexor hallucis brevis along with the flexor hallucis longus muscles are important for the final phase of walking. Flexion of the big toe during the final phase of gait generates a 'push off' lever action of the big toe while walking/running.

The other 'layer' three muscles are the adductor hallucis (adduction of the big toe) and the flexor digiti minimi (flexion of the little toe). Since these have somewhat limited role as we do not have prehensile toes, we will not dissect these muscles.

49. The fourth layer of muscles is formed by the  interossei muscles.
    (Figure523) (Figure 524) (Netter526) (Netter527)

We will not dissect these. 

There are three plantar interossei and four dorsal interossei. These are involved in adduction and abduction of the toes respectively, though functionally this movement is limited relevance in toes compared to fingers.

CHECKLIST

Skeletal Structures

Tibia
    Medial malleolus

Fibula
    Lateral malleolus

Foot
    Talus
    Calcaneus
        Sustentaculum tali
        Calcaneal Tuberosity
    Navicular
    Cuboid
    3 cuneiforms
    Metatarsal bones
    Phalanges (proximal, middle, distal)

Soft Structures

Fascia lata

Sural nerve

Small saphenous vein

Crural fascia

Superficial posterior compartment of the leg
    Gastrocnemius muscle
        Lateral and medial heads
    Soleus muscle
    Plantaris muscle & tendon
    Calcaneal (Achilles') tendon
Deep posterior compartment of the leg
    Flexor digitorum longus muscle & tendon
    Flexor hallucis longus muscle & tendon
    Tibialis posterior muscle & tendon
    Tibial nerve
    Posterior tibial artery & vein
    Fibular artery & vein

Flexor retinaculum

Plantar aponeurosis

First layer
    Flexor digitorum brevis muscle and tendon

Second layer
    Tendon of flexor digitorum longus
    Tendon of flexor hallucis longus
    Quadratus plantae muscle

Third layer
    Flexor hallucis brevis muscle