Dynamic Stabilisers

For Medical Students

Structure and Location

The quadriceps is the dynamic stabiliser of the patella. Its four parts pull in different directions and the patella goes where their sum takes it.

Rectus femoris and vastus intermedius pull almost straight up the thigh.

Vastus lateralis pulls up and laterally. Conventional teaching gives its fibre angle as about 30 degrees to the long axis of the femur.

Vastus medialis has two parts. The longus fibres run at about 15 degrees and the oblique fibres, usually called vastus medialis obliquus, run at about 50 degrees, which is close to transverse.

The oblique fibres attach to the medial border of the patella and to the medial patellofemoral complex, which 2.12.3 covers.

Relations

Vastus medialis obliquus overlies the medial patellofemoral ligament and partly attaches to it, which 2.12.2 covers.

Vastus lateralis blends into the lateral retinaculum, so its pull reaches the patella through fascia as well as through tendon. 2.12.4 covers that.

All four parts converge into the quadriceps tendon above the patella.

Below the patella, the patellar tendon carries the whole force to the tibial tubercle. 2.2.5 covers the tubercle.

Function

The lateral pull from vastus lateralis and the medial pull from the oblique fibres have to balance for the patella to track in the groove.

Because the oblique fibres run almost transversely, they restrain rather than extend. They are the counterweight to the lateral vector.

The whole group loses advantage in early flexion, which is the same range in which the patella is not yet in the groove. Soft tissue, bone and muscle are all at their weakest in the first thirty degrees.

Clinical Relevance

Wasting after injury affects the oblique fibres first and most visibly, which is why the medial bulge above the kneecap disappears early.

A balance problem can come from either side: weak medial fibres or a tight lateral retinaculum produce the same net result.

Dynamic stabilisers are the part of patellar stability that can be changed without an operation, which is why rehabilitation is the first treatment in Section 8.

MRCS Level

Structure and Location

Distinguish the oblique fibres anatomically rather than by name alone. They arise from the adductor magnus tendon and the medial intermuscular septum, not from the linea aspera, and they insert into the upper third to half of the medial patellar border.

That origin is why the oblique fibres are often described as a separate muscle. They have a distinct origin, a distinct fibre angle and often a distinct nerve branch.

Vastus lateralis has an analogous oblique portion arising from the lateral intermuscular septum, so the asymmetry is between two oblique portions rather than between a medial oblique and nothing.

The gluteal and hip muscles belong in this account too, because femoral rotation changes the direction the groove points. 3.12.7 covers dynamic valgus.

Relations

The oblique fibres lie superficial to the medial patellofemoral ligament and partly attach to its upper surface, so a medial approach passes muscle before ligament.

The lateral oblique fibres blend with the lateral retinaculum and the iliotibial band's expansion.

The suprapatellar pouch lies deep to the converging tendon.

The saphenous nerve and its branches lie medial and superficial to the muscle, which 2.9.7 covers.

Blood Supply and Innervation

Blood supply to vastus medialis is from the descending genicular artery and the femoral artery; to vastus lateralis from the lateral circumflex femoral artery.

All four parts are supplied by the femoral nerve, with the oblique fibres of vastus medialis often carried by a separate branch. That separation is the anatomical basis for their weakening ahead of the rest of the quadriceps.

Conventional teaching reports that oblique fibre wasting is measurable within days of an effusion, through reflex inhibition rather than disuse.

Function

The net patellar vector is lateral in most knees, because the lateral pull is larger and the quadriceps angle adds to it. The oblique fibres and the medial soft tissue oppose it. 3.12.6 covers the quadriceps angle.

The oblique fibres cannot overcome a bony problem. Their line of pull is almost transverse and their cross sectional area is small.

Hip abductor and external rotator weakness allows the femur to rotate internally, which moves the groove rather than the patella, and the result looks the same on examination. 3.12.7 covers it.

Clinical Relevance

Examine hip abductor strength and single leg control when the patella is unstable, because the groove's position is set above the knee.

Expect the oblique fibres to waste first and recover last, and record thigh circumference rather than relying on inspection.

Consider the lateral side as well as the medial, since the imbalance can come from either.

Treat dynamic stabilisation as the modifiable part of patellar stability. Bone and ligament are not modifiable without an operation.

FRCS Level

Relations

The oblique fibres' attachment to the medial patellofemoral complex means muscle and ligament are mechanically coupled, so a reconstruction sits under a muscle that also tensions it.

The lateral oblique fibres and the retinaculum are continuous, which is why a lateral release divides a muscular expansion as well as a ligament.

The hip is in the chain. The groove's orientation is set by femoral rotation, so patellar tracking is a two joint problem in the way 2.11.1 describes for the iliotibial band.

Function

Treat dynamic stabilisation as setting the position from which the passive restraints are loaded. Good muscular control means the checkrein is rarely reached; poor control means it is reached often.

The oblique fibres are a restraint and not a corrective force. Expecting them to centralise a patella that is laterally positioned for bony reasons is expecting the wrong thing of a small transverse muscle.

Rehabilitation changes the loading of the passive structures without changing them, which is why it works in some patients and not in others, and the anatomy predicts which.

Clinical Relevance

Assess the hip and the trunk before concluding that a patellar problem is local.

Expect rehabilitation to help where the passive anatomy is near normal and to help less where it is not, and say which case a patient is in.

Consider the coupling between the oblique fibres and a reconstructed ligament when planning rehabilitation after reconstruction. Section 8 carries the programmes.

Avoid describing selective strengthening of the oblique fibres as achievable in isolation. The anatomy shares a nerve and a tendon with the rest of the muscle.