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Posterior cruciate ligament (PCL)

Anterior cruciate ligament (ACL)

Lateral

collateral ligament (LCL)

Lateral meniscus

Semi- membranosus

Medial collateral ligament (MCL)

Figure 2: Main ligaments of the knee (viewed from the posterior aspect)

This rotation is to allow the knee to ‘unlock’ before flexion can occur.

Muscle movements Different muscle groups are involved in movement of the knee. l Knee flexion - hamstring muscles ie. semi-tendinosus, semi-membranosus and biceps femoris with some help from the accessory muscle, popliteus, which unlocks the knee prior to flexion (see Fig.5). l Knee extension - quadriceps muscles ie. rectus femoris, vastus lateralis, vastus medialis and vastus intermedialis (see Fig.6).

The gastrocnemius muscle also aids knee flexion when the foot is fixed. Man can stand straight upright on his legs because the facets of the tibia and femur lock to stabilise the joint in full extension, minimising muscle activity.

Joint stability The stability of the knee joint is a due to the array of ligaments connecting the tibia and femur and the very strong muscles surrounding the joint which are activated in response to tensions within the ligaments.

It is the ligaments, which not only hold the two bones together, but also tighten when they are stretched, firing off stretch receptors, which allows the spinal reflexes to contract the appropriate muscle to avoid excess movement.

This is the main reason why arthritis occurs if a patient has a deficient ACL ligament after injury, because the disarticulation and excess movement within the knee leads to minor trauma and ultimately osteoarthritis.

24 Extension Figure 3: Flexion and extension of the knee Semi- tendinosus Figure 5: Muscles responsible for knee flexion

Vastus intermedialis (sits under rectus femoris)

Rectus femoris

Biceps femoris Flexion

External rotation

Internal rotation

Vastus lateralis

Vastus medialis

Figure 4: Rotation of the knee joint

As in the shoulder, eccentric movements of the prime movers slow the movement of the knee. The two major muscle groups (quadriceps and hamstrings) oppose each other, before reversing roles in the gait cycle. The recurring theme of stabilising muscles having to maintain effective joint movement/articulation before the prime movers can work efficiently, also applies to the knee.

In the knee the vastus medialis (VM) and vastus medialis obliquis (VMO), play key roles in stabilising the patella, to allow the quadriceps to work effectively. Weakness of VM and VMO can be a cause of patella- femoral maltracking which is in turn a cause of anterior knee pain (AKP).

Conclusion As one of the primary load-bearing joints of the body, it is essential to understand the anatomy of the knee joint and how this anatomy translates from form into function. This is particularly important in terms of reducing the risk of injury and preserving healthy function of the joint.

Figure 6: Muscles responsible for knee extension

This article has been adapted from a series of anatomy refreshers published in sportEX medicine for which sportEX was nominated as a finalist in the 2010 Digital Magazine Awards. Visit www.sportex.net

THE AUTHOR

Dr Simon Kay, MBBS MRCGP DRCOG DIPL and MSc Sports medicine, is a member

of FSEM, BASEM, BMAS SOC and is Honoury Education Fellow at Exeter University. He is a full-time GP and sports physician based in Devon. He teaches students, GPs and sports therapists and has been medical officer to various rugby clubs. For more information visit www.sports-doctor.co.uk

With thanks to Primal Pictures for allowing us to use their superb animations and graphics. For more information please visit www.primalpictures.com

TheThe REPS Journal 2009;00(Month):00-00 REPs Journal 2010;19(December):23-24