Vastus Medialis Anatomy

Origin. The muscle arises from a continuous line down the medial femur:

• Lower part of the intertrochanteric line
• Spiral (pectineal) line
• Medial lip of the linea aspera
• Medial supracondylar line, extending almost to the adductor tubercle
• Tendon of adductor magnus and the medial intermuscular septum

That origin reaches further distally on the femur than any other quadriceps head, which is why the vastus medialis is the muscle that must be dealt with in any medial knee arthrotomy.

Insertion. The main bulk forms the medial portion of the quadriceps tendon and inserts on the medial aspect of the patellar base. The VMO is the distal oblique segment. Its fibres run at approximately 50-55 degrees to the femoral shaft, against roughly 15-20 degrees for the proximal, longitudinal fibres of the vastus medialis longus.

The VMO inserts on the medial border of the patella, extending approximately 1.5-2 cm distal to the patellar base, commonly quoted as the proximal third to proximal half of the border. That insertion is the reference for the apex of a midvastus split and for medial reefing.

The layer over the MPFL. Fibres continue into the medial retinaculum and blend with the superficial layer overlying the medial patellofemoral ligament (MPFL), which runs between the adductor tubercle region and the superomedial patella. Because the VMO aponeurosis lies superficial to the MPFL, the ligament is found by elevating or retracting the VMO in MPFL reconstruction, and it is preserved or repaired in an arthrotomy. A distal expansion reaches the medial tibial condyle and the medial capsule.

Two landmarks. The adductor tubercle marks the distal limit of the origin along the medial supracondylar line. It is the landmark for the MPFL femoral origin, which lies just distal and posterior to it, and for the distal extent of a subvastus elevation. The medial intermuscular septum is both origin and posterior boundary; it is the plane elevated in the subvastus approach, and the adductor canal contents lie deep to it.

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The nerve. The nerve to vastus medialis comes from the posterior (deep) division of the femoral nerve, root values L2, L3, L4. It descends within the adductor (subsartorial) canal on the deep surface of the muscle, accompanied by the saphenous nerve and the femoral artery and vein, then leaves the canal to enter the muscle, giving multiple branches.

How it enters the muscle. A long branch runs distally on the deep (posterior) surface, sending branches into the muscle along its length and reaching the region of the VMO. This distal, longitudinally running innervation is the reason a midvastus split must be limited in length: a split extended too far proximally risks the branches supplying the VMO.

Regional supply. Several anatomical studies describe multiple independent branches to different portions of the vastus medialis, with a discrete branch or branches to the VMO segment, an anatomical basis for a degree of independent regional control. Thiranagama's dissections, summarised with the evidence below, traced the medial branch to L1, L2 and L3, but that root tracing rests on only three cadavers, too few to establish it as typical.

Root-value cross-check. Knee extension and the patellar reflex are both L3/L4. Quadriceps weakness with numbness of the anteromedial leg and medial border of the foot suggests a lesion of the nerve to vastus medialis and the saphenous nerve together, not a root lesion; the localisation is set out under Other Pathology.

The dominant supply. The descending genicular (supreme genicular) artery arises from the femoral artery in the adductor canal, just before the femoral artery passes through the adductor hiatus. It gives:

• A muscular (descending) branch that supplies the vastus medialis
• A saphenous branch accompanying the saphenous nerve
• The articular (medial superior genicular / transverse) branch to the medial femoral condyle and the peripatellar ring

Muscular branches of the femoral artery and of the profunda femoris supply the proximal and middle thirds. The medial superior genicular artery supplies the distal muscle and the VMO; it lies distally at the medial femoral condyle, is met during distal exposure, and contributes to the peripatellar anastomotic ring.

The vessel in the subvastus plane. The descending genicular artery is encountered at the deep proximal corner of the subvastus plane, emerging from the adductor canal roughly where the muscle's origin reaches the medial supracondylar line. It is the vessel that catches out the surgeon performing a subvastus approach for the first time, with brisk arterial bleeding from the deep proximal angle of the wound. Identify it and ligate or clip it deliberately during elevation; it is expendable.

Why not to sacrifice it casually. The descending genicular artery is the pedicle of the medial femoral condyle corticoperiosteal (vascularised bone) flap, used for recalcitrant non-unions. Preserve it in a patient who may need that reconstruction.

The patella. The peripatellar anastomotic ring is fed by the superior and inferior, medial and lateral genicular arteries plus the anterior tibial recurrent artery. A medial arthrotomy divides the medial contributors, and a concurrent lateral release divides the superior lateral genicular artery: that combination has been implicated in patellar avascular necrosis, and combined with fat pad excision it has been implicated in avascular necrosis and patellar fracture.

Superficial (medial and anterior). Distally, sartorius overlies the muscle's deep medial border and, with the vastoadductor membrane, forms the roof of the adductor canal. Anteromedially lie the fascia lata and subcutaneous tissue, and in the subcutaneous plane over the anteromedial knee run the great saphenous vein and the infrapatellar branch of the saphenous nerve.

Deep (posterior and lateral). Vastus intermedius lies laterally over the anterior femoral shaft, separated from the vastus medialis by a plane that is often indistinct distally, and the femoral shaft and periosteum lie medially. Distally the adductor canal and its contents lie deep to the muscle, described with the block under Clinical Relevance. Posteriorly lie adductor longus and adductor magnus, with the medial intermuscular septum between the compartments.

Distal. The suprapatellar pouch and the medial capsule lie beneath the distal fibres, separated from the synovium by only a thin layer of fat, which is why subvastus elevation reaches the joint so directly.

The traditional model, now substantially revised. VMO weakness or delayed VMO activation was held to cause lateral patellar tracking, which raised lateral facet pressure and caused pain, so selective VMO strengthening was prescribed. VMO wasting in patellofemoral pain is now understood to be predominantly secondary, driven by arthrogenic muscle inhibition, deconditioning and altered movement strategies.

You cannot isolate the VMO. No exercise has been shown to reliably recruit it preferentially over the vastus lateralis, and electromyographic studies show the two activate together. Studies of the commonly recommended manoeuvres (terminal extension, adduction with extension, hip adduction added to squats, taping) have not demonstrated reproducible preferential VMO recruitment.

What does work.

• Whole-quadriceps strengthening
• Hip abductor and external rotator strengthening
• Motor control and movement retraining
• Load management
• Where indicated, taping or bracing for short-term symptom relief, and orthoses for selected patients

The proximal contribution. Femoral internal rotation and adduction during weight-bearing effectively move the trochlea under the patella, producing lateral maltracking that no amount of VMO training corrects.

Structural risk factors to assess before blaming the VMO.

First-time lateral dislocation. The MPFL is the structure that fails, most commonly at its femoral attachment, and the VMO aponeurosis overlying it is disrupted. Osteochondral fragments from the medial patellar facet or the lateral femoral condyle must be sought. Management is generally non-operative unless there is an osteochondral fragment requiring fixation, an irreducible dislocation or a large loose body, with rehabilitation targeting quadriceps activation, hip control and graded return.

Recurrent instability. MPFL reconstruction restores the passive restraint, with the graft placed deep to the VMO aponeurosis. The femoral origin lies in a small depression just distal and posterior to the adductor tubercle, between the tubercle and the medial epicondyle; radiographically it is the Schöttle point on a true lateral. Bony correction (tibial tubercle transfer, trochleoplasty, derotation) is added for the structural abnormalities above.

Medial reefing. Isolated medial reefing (VMO advancement) has largely been superseded by MPFL reconstruction, because of its higher recurrence rates and the risk of medial overload.

The saphenous nerve. The terminal sensory branch of the femoral nerve travels in the adductor canal with the femoral artery and the nerve to vastus medialis. It exits by piercing the vastoadductor membrane and becomes subcutaneous between sartorius and gracilis.

The infrapatellar branch. It arises from the saphenous nerve around the level of the medial knee and passes obliquely across the anteromedial knee, most commonly deep to or through the distal sartorius. It fans into branches supplying the skin over the inferomedial patella, the patellar tendon region and the anteromedial proximal tibia, most of them lying in the subcutaneous plane at and below the joint line and medial to the patellar tendon.

Its direction changes across the knee. In Kerver's cadaveric mapping (below) it runs nearly vertically on the medial side, obliquely medial to the patellar tendon and close to horizontally over the patella and tendon, and no definite safe zone could be identified. The standard longitudinal knee incision crosses it, as does essentially every anteromedial knee approach, and it is the commonest nerve injury in knee surgery.

What the patient notices. Numbness in a patch anterolateral and distal to the incision is extremely common and usually well tolerated once explained. A painful neuroma gives a positive Tinel sign over the nerve's course, allodynia and a well-localised trigger point. It is a recognised cause of persistent anteromedial pain after arthroscopy, arthroplasty, tibial nailing, hamstring graft harvest and medial meniscal repair, and relief with a small-volume local anaesthetic block precisely at the tender point confirms it.

Treatment, in order.

• Explanation and time
• Desensitisation
• Neuropathic analgesia
• Targeted local anaesthetic and steroid injection
• For the refractory case, neurectomy proximal to the neuroma with the stump buried in muscle; counsel that neurectomy trades pain for a defined patch of numbness

Prevention. No incision is safe, only less risky. Where the operation allows a choice, place the incision parallel to the nerve's course at that site: for hamstring graft harvest, an oblique or vertical incision over the pes anserinus placed to run parallel to the nerve reduces risk compared with a transverse incision. Raise full-thickness flaps rather than dissecting within the subcutaneous fat, and consent explicitly; preoperative documentation of any existing deficit protects both patient and surgeon.

The adductor canal. The subsartorial (Hunter's) canal runs from the apex of the femoral triangle to the adductor hiatus in adductor magnus. Its roof is the vastoadductor membrane deep to sartorius, its anterolateral wall the vastus medialis, and its posteromedial wall adductor longus and adductor magnus. It contains the femoral artery and vein, the saphenous nerve and the nerve to vastus medialis, and the descending genicular artery arises within it.

Quadriceps-sparing, not motor-neutral. The adductor canal block is widely used for knee arthroplasty analgesia and is often described as motor-sparing. It is substantially more quadriceps-sparing than a femoral nerve block, which blocks the whole quadriceps, but it anaesthetises the nerve to vastus medialis as well as the saphenous nerve, so some vastus medialis weakness occurs. Jaeger's crossover study in 11 healthy volunteers put numbers on both halves of that statement: mean quadriceps strength fell 8 per cent after adductor canal block against 49 per cent after femoral nerve block, and the 8 per cent was still statistically significant against placebo (p = 0.02).

Where the needle actually goes. Ultrasound studies have shown that many injections labelled adductor canal blocks are actually placed in the distal femoral triangle, proximal to the true canal. Wong found the conventional midthigh insertion point lies a mean 4.6 cm proximal to the start of the canal (range 2.3 to 7.0), and it was proximal to the canal in all 22 volunteers. Define the canal instead by where the medial border of sartorius crosses the medial border of adductor longus, which scales with the patient; half the thigh length does not.

Why the level matters. The more proximal the injection, the greater the chance of blocking additional motor branches and the greater the quadriceps weakness, a genuine falls-risk consideration on the ward after arthroplasty. Whatever the block, assess quadriceps power and use a knee immobiliser or supervised mobilisation for the first ambulation after any lower limb nerve block.

Extensor lag after arthroplasty. Lag is common in the first weeks after a medial parapatellar approach and should improve. Persistent lag beyond three months warrants assessment for VMO tendon dehiscence, quadriceps denervation, or an unrecognised patellar tendon or quadriceps tendon injury.

Quadriceps tendon rupture. The rupture involves the medial (vastus medialis) contribution along with the rest, and the suprapatellar gap and extension lag are the diagnostic features.

Muscle injury and contracture. Strain of the vastus medialis is uncommon relative to the biarticular muscles. A direct contusion on the medial thigh risks myositis ossificans with progressive stiffness, and radiographs show centripetal (zonal) maturation. Post-traumatic or post-surgical fibrosis limits knee flexion and can tether the patella medially; where fibrosis follows an extension contracture, quadricepsplasty may be required.

The denervation pattern. Complete denervation of the vastus medialis produces visible loss of the VMO bulge, terminal extension weakness and a maltracking patella; on MRI there is selective fatty atrophy of vastus medialis with preserved vastus lateralis and rectus femoris.

Localising the lesion. Because the saphenous nerve travels with the nerve to vastus medialis in the adductor canal, isolated vastus medialis atrophy with anteromedial leg and medial foot border numbness is a combination that localises the lesion: it suggests the adductor canal, or the posterior division of the femoral nerve. Causes include compression by the vastoadductor membrane, iatrogenic injury during medial knee surgery or vascular access, and a mass lesion.

The non-surgical differential. Diabetic lumbosacral radiculoplexus neuropathy (diabetic amyotrophy) presents with severe thigh pain and profound proximal quadriceps wasting, including the vastus medialis, and is an important differential.