Instability of the knee joint

Instability of the knee joint: what's that?

The knee joint  is a synovial rolling and sliding joint  which allows the knee to alternate between bending, stretching and rotational movement (rolling and sliding movement). Hereby, intra-articular muscles and various ligaments hold the knee joint in its physiological motion sequence, ensuring stability. The anterior and posterior cruciate ligaments are also particularly important here as they regulate the motion sequence forwards and backwards. The medial and lateral collateral ligament practically keeps the knee “on track” from the side.

As is true for all the joints in the human body, the knee joint is also covered by a joint capsule, which provides the bone with nutrients and fulfils a shock-absorbing function together with the enclosed menisci during motion sequences.

The capsuloligamentous structure can be so damaged due to wear, which primarily affects the menisci, that the knee joint becomes instable.

A so-called complex knee joint instability (e.g. antero-medial instability) is normally referred to when several ligaments fail at the same time.

Causes of instability of the knee joint

Hereditary hypermobility (laxity) of the knee joint is seen seldom and is, for example, observed within the context of the Ehlers-Danlos-Syndrome (hereditary hyperextensibility of the skin and hypermobility of the joints).

The most frequent causes of knee joint instability are (sports) accidents, where predominantly the medial collateral ligament (MCL) and the anterior crucial ligament (ACL) are damaged. If several ligaments are affected, complex instability is incurred.

In rare cases, dislocation of the leg axis – such as varus or valgus malalignment – with additionally incurred cartilage loss result in laxity.

Symptoms of instability of the knee joint

Acute ligament injuries after accidents are observed in more than 90% of the cases. Following abatement of the accident-caused swelling, a subjective feeling of instability exists, which is referred to the giving-way phenomenon in the jargon. The patients concerned have the feeling that the knee slides away or gives way and that already under a low, normal burden such as going upstairs or sprinting to catch a tram. This phenomenon is often also described as “snapping joint”.

Every knee joint which is instable in the long-term (chronic instability) leads to damages to the articular cartilage (gonarthrosis) and meniscus problems. However, patients often don’t notice chronic instability at all. Externally, this can be seen as a weakness or missing manifestation of the quadriceps musculature (thigh musculature).

If the cartilage and meniscus are damaged, chronic instability can repeatedly cause pain and swelling as well as effusion in the knee joint.

Stages of instability of the knee joint

The so-called “drawer test” is most frequently applied for the quantification of the stage of a knee joint instability. When the anterior cruciate ligament is damaged, it establishes hypermobility of the lower leg vis-à-vis the thigh at the front and, in case of damage of the posterior cruciate ligament, at the rear. The precise stability measurement of the knee joint is made using measurement devices (KT-1000, rolimeter, KLT) which are placed on the joint or the bone contours. These instruments are used to record the hypermobility under stress in millimetres (= quantification). An X-ray where the knee joint is clamped and the angle of so-called expandability is measured in degrees provides a further option to establish the degree of instability.

In Medicine, three types of knee instability are distinguished:

1. Simple knee instability: If only one of the elements of the capsule-ligament system is affected, e.g. the lateral collateral ligament.
2. Complex knee instability: If several elements of the capsule-ligament system are affected simultaneously, e.g. various ligaments at the same time with additional capsule damage.
3. Chronic knee instability:This spreads over several years.

Diagnosis of instability of the knee joint

An overall view of the patient is necessary for the diagnosis of instability of the knee joint. Magnetic resonance imaging (MRI) can be applied to establish which ligament is injured. Accident case history and the patient’s complaints are also part of the diagnosis. Normally, a clinical examination using various standardised examination procedures is only possible after the knee swelling has declined. The instability of the knee and the function of the ligaments can be checked using specific tests such as the Lachman test, the pivot shift test or the drawer test. It is only possible to make a clear diagnosis when the triad of imaging, measurement procedures and clinical examinations has been completed. In particular in case of chronic instability, different disorders may overlap. For example, a pronounced valgus malalignment can increase accident-caused instability of the medial collateral ligament. Moreover, leg axis, pelvic obliquity and characteristics of the musculature are significant for diagnosis.

How can instability of the knee be prevented?

Instability of the knee can only be prevented with intense muscle-building and coordination training  . In certain cases, particularly in case of longer-term burdens or load situations with a high risk of injury, wearing a supporting knee support  or brace may be a sensible preventive measure.

What therapy options are available for instability of the knee?

Normally, the therapy is determined by the degree of severity of the knee instability. The patient and physician discuss the therapeutic procedure jointly here.
In case of simple instability of the knee, conservative measures such as physiotherapy together with muscle strength training  often help. Anti-inflammatory painkillers and stabilising knee supports or braces can also provide additional support for the healing process.

If the patient’s psychological stress is high, i.e. the instability of the knee is a huge burden for her/him, an operative ligament reconstruction may be a viable alternative in case of ligament injuries. In any case, this must subsequently be supported with corresponding rehabilitation measures. The operation is more or less without any alternative for active, athletic people. However, the degree of severity and the patient’s feeling are also decisive here.