3.2 Superficial & Deep Posterior Compartments of the Leg

3.2 Superficial & Deep Posterior Compartments of the Leg

Independent Study Guide Notice: Independent study guide by OpenExamPrep. Not sponsored by, endorsed by, or affiliated with the National Board of Podiatric Medical Examiners (NBPME) or the American Podiatric Medical Licensing Examination (APMLE).

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Compartmental Partitioning & Fascial Boundaries

The posterior aspect of the leg contains the largest muscle mass in the lower extremity, partitioned into two distinct osteofascial compartments by the transverse intermuscular septum (deep transverse fascia):

1. Superficial Posterior Compartment: Bounded posteriorly by the investing crural fascia and anteriorly by the transverse intermuscular septum. It houses the three plantarflexors that make up the triceps surae and plantaris.
2. Deep Posterior Compartment: Bounded anteriorly by the posterior surfaces of the tibia, fibula, and the interosseous membrane, and posteriorly by the transverse intermuscular septum. It encloses four deep muscles (popliteus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior) along with the posterior tibial neurovascular bundle.

All muscles in both posterior compartments are innervated by the tibial nerve (roots L4, L5, S1, S2, S3), which represents the larger terminal bifurcation of the sciatic nerve.

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Superficial Posterior Compartment: The Triceps Surae & Plantaris

Functional & Morphological Nuances

• Gastrocnemius: A two-joint (biarticular) muscle composed predominantly of fast-twitch (Type II) muscle fibers designed for rapid, powerful bursts of propulsion during running and jumping. Because it crosses the knee joint, its capacity to plantarflex the ankle is substantially attenuated when the knee is flexed to 90 degrees (which places the gastrocnemius on passive mechanical insufficiency). The lateral head frequently contains a sesamoid bone, the fabella, present within its tendon of origin in 10% to 30% of individuals.
• Soleus: A broad, flat, multipennate, single-joint (uniarticular) muscle situated immediately deep to the gastrocnemius. Composed predominantly of slow-twitch, fatigue-resistant (Type I) muscle fibers, it serves as the essential antigravity muscle maintaining postural equilibrium during upright standing. In addition, its deep intramuscular venous plexuses (soleal venous sinuses) function as the primary "peripheral heart" of the calf, pumping deoxygenated venous blood against gravity toward the femoral-iliac axis during ambulation.
• Tendinous Arch of Soleus (Arcus Tendineus Musculi Solei): A fibrous bridge connecting the tibial and fibular origins of the soleus. Beneath this arch, the popliteal artery (which bifurcates into the anterior and posterior tibial arteries) and the tibial nerve pass safely to enter the deep posterior compartment of the leg.
• Plantaris ("Freshman's Nerve"): Features a tiny, fusiform muscle belly (7–10 cm in length) accompanied by the longest tendon in the human body. The tendon courses obliquely downward between the gastrocnemius and soleus. Due to its glistening, thin appearance, it is notoriously mistaken by novice anatomy students for a peripheral nerve. Acute rupture of the plantaris tendon or strain of the medial gastrocnemius myotendinous junction produces sudden calf pain historically termed "tennis leg."

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The Achilles Tendon (Tendo Calcaneus) & Calcaneal Bursae

Structural Twisting & Biomechanical Storage

The Achilles tendon is the thickest and most powerful tendon in the body, capable of withstanding tensile loads exceeding 8 to 10 times body weight during sprinting.

• 90-Degree Rotational Twist: As the aponeuroses of the gastrocnemius and soleus fuse to form the conjoint tendon, their fibers undergo an internal spiral rotation of approximately 90 degrees. As viewed from posterior:
  • The gastrocnemius fibers (arising anteriorly and proximally) rotate posterolaterally.
  • The soleus fibers rotate posteromedially.
• Significance: This spiraling concentrates elastic strain energy during midstance and releases it rapidly during propulsion, while dampening shear stresses within individual fascicles.

Anatomical Insertion & Associated Bursae

The tendon inserts onto the middle third of the posterior surface of the calcaneus. It is buffered by two distinct synovial bursae:

1. Subcutaneous Calcaneal Bursa: Situated superficially between the posterior skin/subcutaneous tissue and the distal Achilles tendon. Inflammation caused by rigid footwear counters produces superficial calcaneal bursitis.
2. Retrocalcaneal Bursa: Situated deep between the anterior surface of the distal Achilles tendon and the posterosuperior superior calcaneal tuberosity. Friction against an enlarged posterosuperior calcaneal prominence produces Haglund deformity ("pump bump") and retrocalcaneal bursitis.

                                    [Achilles Tendon]
                                           │
                                           │ (Watershed Zone: 2-6 cm proximal)
                                           │
                 Subcutaneous              │             Retrocalcaneal
                 Calcaneal Bursa           │             Bursa
                   (Superficial)           │             (Deep to tendon)
                          *                ▼                    *
               [Skin] ────┼─────── [Achilles Tendon] ───────────┼────── [Calcaneus]
                                           │                        (Posterosuperior
                                           ▼                         tuberosity /
                             [Middle 1/3 Calcaneal Tuberosity]       Haglund prominence)

The Avascular Watershed Zone & Tendon Rupture

• Watershed Zone: Angiographic and microvascular studies demonstrate a localized region of relative hypovascularity situated 2 to 6 cm proximal to the calcaneal insertion. This watershed zone is nourished primarily by tenuous paratenon vessels (rather than true vincula or endosteal branches).
• Pathology: Repetitive microtrauma and chronic ischemia within this zone cause non-insertional Achilles tendinopathy, characterized by mucinous degeneration, disorganized collagen, and lack of inflammatory cells (angiofibroblastic tendinosis).
• Acute Rupture & The Thompson Test: Sudden eccentric loading of a dorsiflexing ankle with an extended knee (e.g., pushing off in racquet sports) can cause complete catastrophic rupture through the watershed zone. Patients report feeling struck in the calf from behind with an audible "pop."
  • Thompson (Simmonds-Thompson) Test: With the patient placed prone and the feet dangling freely over the examination table, the examiner squeezes the fleshy middle third of the calf. A normal, intact tendon produces involuntary passive plantarflexion. Absence of plantarflexion confirms complete Achilles tendon discontinuity.

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Deep Posterior Compartment: Myology & Paths

The deep compartment houses four muscles. One (popliteus) acts at the knee; the other three (FHL, FDL, and tibialis posterior) cross the ankle into the plantar foot.

The Popliteus: "Key to the Knee"

The popliteus occupies an exceptional anatomical position:

• Intracapsular Origin: Arises from a pit at the anterior end of the popliteal groove on the lateral epicondyle of the femur. Its tendon passes inside the fibrous capsule of the knee joint but remains extrasynovial (separated from the synovial cavity by a synovial reflection). It pierces the posterior capsule beneath the arcuate popliteal ligament.
• The Screw-Home Mechanism: When the knee reaches terminal extension (0 degrees), the tibia undergoes automatic external rotation relative to the femur (or the femur internally rotates on the tibia), locking the joint into maximal osseous congruency.
• Unlocking Action: To initiate flexion from full extension, the popliteus contracts. In weight-bearing (closed kinetic chain), it laterally (externally) rotates the femur on the fixed tibia. In non-weight-bearing (open chain), it medially (internally) rotates the tibia on the femur. This action releases the screw-home lock.

Master Knot of Henry & Tarsal Tunnel Crossings

At the medial ankle, the tendons of the deep posterior compartment enter the foot through the tarsal tunnel deep to the flexor retinaculum (laciniate ligament). Their spatial sequence from anterior to posterior is universally remembered by the classic mnemonic:

"Tom, Dick And Very Nervous Harry"

1. T - Tibialis posterior tendon (anterior-most, immediately abutting medial malleolus)
2. D - Flexor digitorum longus tendon
3. A - Posterior tibial artery
4. V - Posterior tibial vein(s)
5. N - Tibial nerve
6. H - Flexor hallucis longus tendon (posterior-most, grooving the posterior process of the talus between its medial and lateral tubercles)

Upon entering the second layer of the plantar foot at the level of the naviculocuneiform articulation, the FDL and FHL tendons cross one another at the Master Knot of Henry:

• Spatial Crossing: The FDL crosses superficial (plantar) to the FHL tendon.
• Tendinous Slip: The FHL typically sends a strong fibrous slip to the FDL tendon, ensuring that forceful flexion of the hallux mechanically augments flexion of digits 2 and 3.

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Adult Acquired Flatfoot Deformity (PTTD)

The tibialis posterior tendon is the principal dynamic inverter of the subtalar joint and the primary dynamic stabilizer of the medial longitudinal arch. As the foot pronates during stance, the tendon acts eccentrically to decelerate internal tibial rotation and midtarsal joint pronation.

                                 [Tibialis Posterior Muscle]
                                             │
                                             ▼
                              [Course Behind Medial Malleolus]
                                             │ (Hypovascular Zone)
                                             ▼
                               [Navicular Tuberosity Insertion]
                                             │
                                             ├──> Primary: Navicular Tuberosity
                                             └──> Expansions: Cuneiforms (1-3),
                                                  Cuboid, Metatarsals (2-4),
                                                  Sustentaculum Tali
                                                  (EXCLUDES Talus & 5th Metatarsal)

Stages of Posterior Tibial Tendon Dysfunction (Johnson & Strom Classification with Myerson Modification)