Dynamic Stabilisation

For Medical Students

Structure and Location

Four muscle groups act as dynamic stabilisers on the medial side of the knee.

Semimembranosus is the largest contributor. Its five arms tension the posteromedial corner directly, and 2.9.5 covers them.

The pes anserinus group is sartorius, gracilis and semitendinosus, three tendons from three different nerves that insert together on the anteromedial tibia below the joint line.

Vastus medialis, and its oblique fibres in particular, acts on the patella rather than on the medial ligaments, which makes it a medial stabiliser of a different joint surface. 2.12.5 covers it.

The medial head of gastrocnemius crosses the back of the knee and arises just behind the posterior oblique ligament's femoral attachment.

Relations

The pes tendons lie in layer one, superficial to the collateral ligament, with a bursa between them and it.

Semimembranosus lies deep to semitendinosus and medial to the medial head of gastrocnemius, with a bursa at each interface.

Vastus medialis obliquus runs almost transversely into the medial border of the patella, which is why it pulls the patella medially rather than upwards.

Function

A ligament sets a limit. A muscle decides how often that limit is reached, which is the difference between a static and a dynamic stabiliser.

The pes group resists valgus and internal rotation as the knee flexes, acting in the same direction as the collateral ligament.

Semimembranosus tensions the posteromedial corner and pulls the posterior horn of the medial meniscus backwards.

The medial head of gastrocnemius resists hyperextension and contributes to posterior stability. 3.9 covers how much each contributes.

Clinical Relevance

A knee with intact ligaments and poor muscle control is not a stable knee. The anatomy says why, and Section 8 covers what is done about it.

The pes tendons cross the field of a medial approach, so a medial incision passes three tendons before reaching a ligament.

Because the pes group is supplied by the femoral, obturator and sciatic nerves, the three tendons do not fail together. One can be weak while the others work.

MRCS Level

Structure and Location

Give the pes anserinus in order from front to back: sartorius, gracilis, semitendinosus. The order matters because a graft harvest takes the middle two and the order says what is being passed.

Conventional teaching places the pes insertion about 4 cm below the joint line, superficial to the distal attachment of the collateral ligament, with the pes bursa between them.

Vastus medialis obliquus is distinguished from the rest of vastus medialis by fibre angle. Conventional teaching gives its fibres an angle of the order of 50 degrees to the long axis of the femur, against about 15 degrees for the longus fibres.

The medial head of gastrocnemius arises from a facet just behind and above the medial epicondyle, adjacent to the posterior oblique ligament's attachment.

Relations

The saphenous nerve emerges between sartorius and gracilis, so the pes is both a landmark for the nerve and a hazard to it.

The pes bursa separates the tendons from the collateral ligament's distal attachment. That is the interval a harvest works in.

Semimembranosus and the medial head of gastrocnemius are separated by a bursa that communicates with the joint in a substantial minority of knees.

Vastus medialis obliquus has no attachment to the medial collateral ligament. Its medial attachment is to the adductor magnus tendon and the medial intermuscular septum.

Blood Supply and Innervation

The three pes tendons carry three nerve supplies: sartorius from the femoral nerve, gracilis from the obturator nerve, semitendinosus from the tibial division of the sciatic nerve.

Semimembranosus is supplied by the tibial division, and the medial head of gastrocnemius by the tibial nerve itself.

Vastus medialis is supplied by the femoral nerve, with the oblique fibres often carried by a separate branch, which is the anatomical basis for them weakening before the rest of the quadriceps.

Blood supply to the pes insertion is from the medial inferior genicular artery and the descending genicular artery.

Function

Dynamic restraint is anticipatory as well as reactive. Muscle tension that is present before a load arrives protects a ligament that would otherwise take it alone.

The pes group produces internal rotation and resists valgus, so it works with the collateral ligament rather than against it.

Vastus medialis obliquus resists lateral patellar translation. Its line of pull is almost transverse, so it restrains rather than extends.

Loss of any one dynamic stabiliser is partly compensated by the others, which is why deficits here are harder to detect than ligament injuries.

Clinical Relevance

Examine muscle bulk and control as part of assessing medial stability. A ligament examination alone describes half the structure.

Expect a graft harvest from gracilis and semitendinosus to alter dynamic medial restraint, which is anatomy rather than a complication.

Protect the saphenous nerve where it emerges between sartorius and gracilis, because the interval used for harvest is the interval the nerve uses.

Consider separate innervation when a single pes tendon is weak. The three do not share a nerve.

FRCS Level

Relations

The harvest interval and the nerve interval are the same interval. That single fact accounts for most saphenous nerve injuries at the knee.

The pes bursa is the plane between the dynamic and static medial restraints. Working in it keeps the two apart; working through it does not.

The medial head of gastrocnemius and the posterior oblique ligament share an area of femoral bone, so a dynamic structure and a static one compete for the same fixation point. 2.9.3 covers it.

Function

Treat medial stability as the sum of a static limit and a dynamic tension, and state which one a finding refers to. A knee that opens under anaesthetic and holds when awake is a knee whose dynamic component is working.

Harvesting two of the three pes tendons removes dynamic internal rotation restraint from a knee that may also have lost static restraint. The anatomy makes that worth stating before the harvest rather than after.

Vastus medialis obliquus is a patellar stabiliser and not a tibiofemoral one, so it does not belong in an account of medial collateral function. 2.12.5 holds it.

Clinical Relevance

Consider graft source when the medial side is already injured, because the pes is both a graft and a stabiliser.

Assess dynamic restraint under anaesthetic and awake, and record the difference. It is the one measurement that separates the two components.

Protect the saphenous nerve by finding it rather than by avoiding its territory. The interval it runs in is the interval the operation uses.

Avoid attributing medial laxity to ligament alone in a wasted limb. The anatomy offers two explanations and only one of them is treated in theatre.