Section 1: Epidemiology and aetiology
The knee is the largest joint in the human body. Complicated forces are exerted through the knee during various activities, making it susceptible to a range of acute and chronic injuries.
A recent epidemiological review in the US revealed the incidence of knee injuries to be 2.29 per 1,000 population.1 The highest rate of injuries was in the 15-24 years age group, whereas knee injuries were least seen in under-fives.
Sports and recreational activities probably cause the greatest numbers of knee injuries, although a significant proportion of these may occur from relatively trivial injuries in the older age group.
The knee is not a simple hinge joint; it has some freedom of movement in rotation. The complex interplay of ligaments and intra-articular structures during knee movements results in different injury patterns. A good working knowledge of knee anatomy and pathology, as well as the common pitfalls, is essential to ensure thorough assessment and optimal management.
There is no comprehensive classification of soft tissue knee injuries. For the purpose of systematic enumeration, a list of the anatomical soft tissue structures that can be involved in injuries of the knee is as follows:
- Extensor mechanism comprising parts of the quadriceps muscle, the quadriceps tendon, the patella, extensor retinaculi and patellar tendon, followed by its insertion into the tibial tuberosity.
- Ligaments: medial collateral ligament (MCL) and lateral collateral ligament (LCL); anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL).
- Menisci (medial and lateral).
- Capsular structures and associated ligaments.
Some injuries may be isolated to one structure, whereas others may be a combination of soft tissue injuries. The classic injury is O'Donoghue's triad of torn MCL, torn meniscus and torn ACL.
Section 2: Making the diagnosis
Knee arthroscopy revealing a torn anterior cruciate ligament (Photograph: Author image)
Femoral notch and tibial spine after debridement of ACL stump (Photograph: Author image)
In the A&E setting, while fractures are usually diagnosed, soft tissue injuries may be missed, especially by non-specialist or junior clinicians. Also, some patients may present to their GP days after an obscure injury, without realising that some injuries might have serious consequences.
It is said that most injuries can be diagnosed from a good history and that examination should simply confirm the diagnosis. A detailed account of what happened at the time provides clues to the structures involved and the severity of the injury.
Injuries to ligaments such as the ACL or MCL most commonly occur during contact sports. The classically described, relatively rare, injury to the PCL occurs with a direct impact to the front of the proximal tibia when it is forcibly pushed back (for example, dashboard injury during a car accident).
Pain and swelling
Pain from mechanical causes, for example, pulling of a torn meniscal flap, is typically sharp, shooting, reasonably well localised, intermittent and usually related to a weight-bearing activity involving a twisting action of the knee.
Arthritic pain, on the other hand, is usually an aching or burning pain. Bone on bone arthritis can cause occasional sharp pains on a background ache. It should always be kept in mind that a concurrent pathology of the hip joint may cause referred pain to the knee and confuse the examiner.
Swelling appearing soon after injury usually signifies bleeding in the joint (haemarthrosis), which results from a major ligamentous disruption or major meniscal injury and deserves careful evaluation. Swelling that builds up over hours suggests an effusion.
Other features may include 'giving way' (suggesting ligament injury) or 'locking and clicking' (associated with meniscal injury). What patients describe as dislocation is most commonly a patellar dislocation. True tibiofemoral dislocations are very severe injuries and commonly associated with vascular injury. Previous injuries could have a bearing; for example, a previous patellar dislocation increases the susceptibility for recurrence.
A thorough examination of the knee is mandatory after obtaining a history. Evaluation for tenderness helps to localise the injury; for example, joint line for meniscal injury, areas of attachment of the collateral ligaments or medial border of the patella in patellar dislocations. Evaluation of the extensor mechanism (that is, the ability to perform a straight leg raise) is very important. With extensor mechanism disruption, a gap is often felt on palpation.
Most injuries result in joint stiffness due to pain and effusion. Locking (inability to straighten the knee) is felt as a sudden mechanical block to extension. It is a classic symptom described with a bucket handle meniscal tear. It may also result from impingement of a torn ACL stump or loose body.
Inability to straighten the knee, if it persists beyond two to three weeks, requires relatively urgent specialist attention. Following initial assessment, it is prudent to have a review after one to two weeks, when acute symptoms may have resolved.
When to perform tests
Collateral ligament injuries are evaluated with varus and valgus stress tests. Partial tears are painful to stress testing, but complete tears might be painless.
Drawer and Lachman tests are described for the evaluation of cruciate ligament deficiencies. These and the pivot shift test are difficult to elicit when patients keep their knee guarded. It is necessary for the patient to be relaxed, to allow the manoeuvres. McMurray's test, described for menisci, is known to have little sensitivity and specificity. Joint line tenderness is often the only sign.
Section 3: Managing the condition
Dislocated patella: incomplete healing can lead to chronic instability (Photograph: Zephyr/SPL)
CT allows bony anatomy to be identified and is of value in cases of fracture. MRI provides details of bone and soft tissue.
MRI is more sensitive for soft tissue injuries and may show a wider area of bone bruising. It may not be possible to obtain good images, unless the patient can lie still for 15-30 minutes.
MRI may be contraindicated in patients with pacemakers. In these cases, a CT arthrogram may be undertaken. This requires injection of contrast into the joint followed by CT scan.
It is best to obtain a relatively urgent MRI in suspected ligamentous disruption. There may be a few weeks' wait for this unless the patient is admitted to hospital. Reassessment of the patient in a week or two (after the acute phase of injury settles) is therefore vital. A management plan can be instituted after imaging confirms the diagnosis.
Extensor mechanism disruption must be diagnosed clinically and referred to A&E or an orthopaedic department.
Urgent referral is necessary even if there is doubt regarding continuity of the extensor mechanism. Delayed intervention makes the procedure more difficult and the outcome poor. The knee must be protected and braced postoperatively.
The quadriceps wastes within days of injury and takes a long time to recover, so splinting of the knee in extension is not recommended beyond the acute phase. In collateral ligament injury, a hinged knee brace is provided and flexion restricted.
A patellar dislocation may have relocated when the patient presents. A swollen knee and tender medial retinaculum suggests this injury and MRI will confirm it. This may heal after a few weeks of restricted flexion and rehabilitation. Incomplete healing can predispose to chronic patellar instability.
In cases of ACL injury, it is important to start physiotherapy following the MRI, so as to regain the full range of movement and restrict muscle wasting. If rehabilitation has not managed to attain full extension, impingement by torn ACL is possible and relatively urgent arthroscopy and debridement of torn ligament may be necessary.
Reconstruction of the ACL is planned only after attainment of a full range of movement and if the patient has symptoms of giving way, with signs of instability.
Knee joint aspiration
There are a few indications for carrying out aspiration of a knee joint after injury, for example, a tense painful haemarthrosis. This procedure must be carried out in a sterile environment.
The only cases of meniscal tears that require relatively urgent intervention are knees that remain locked, with large bucket handle tears.
If the tear is in the peripheral zone, it may be repaired arthroscopically by suturing. Other tears are treated with arthroscopic partial meniscectomy, when only the torn unstable part is removed.
Not all meniscal tears require intervention, unless they are symptomatic. It is not uncommon to see an MRI report of meniscal tears with no symptoms or signs. On the contrary, there are cases with classical meniscal symptoms and normal MRI. Cases must be managed on clinical grounds because MRI has a limited sensitivity for detection of meniscal tears.2
Section 4: Prognosis
Following repair of the extensor mechanism, knee flexion needs to be slowly increased. Return to full function will be gradual, over four to six weeks. Residual weakness in the quadriceps muscle group may take a much longer time to resolve.
After ACL reconstruction, rehabilitation continues for at least nine months and return to contact sport cannot be considered before then. Bracing is no longer undertaken after routine ACL reconstruction because starting range of movement exercises immediately after the operation does not strain the reconstructed ligament.
The graft undergoes a process of gradual incorporation at the new site and becomes weaker at about two months postoperatively, before gradually regaining strength. Graft failure occurs in 5-10% of patients.
Partial tears of collateral ligaments heal with bracing over six to eight weeks. Sensitivity over the area can persist for a few months, especially if there has been bone bruising. A steroid injection may relieve pain if symptoms persist for longer.
Recovery from arthroscopic meniscectomies is rapid, with return to normal activity possible in two to three weeks. However, in meniscal repair with suturing, bracing and non weight-bearing are generally instituted for four to six weeks.3
An absent meniscus predisposes the affected knee compartment to greater impact and development of arthritis in the long term. Any remaining amount of meniscal tissue may afford some shock absorption. The principle in carrying out partial meniscectomy is to retain as much of the tissue as possible.
An absent ACL results in abnormal pivoting of the knee and predisposes to meniscal tears.4 Younger patients and those who intend to participate in sports are recommended to have a ruptured ACL reconstructed. Many patients with ACL injuries do not require reconstruction, however, especially if they do not experience symptoms of giving way, do not take part in high-demand activities and are in the older age group.
Section 5: Case study
A 22-year-old man was playing football when he was injured in a tackle. He described the injury as: 'My foot was caught and hit the other player's foot while my body twisted and fell.'
The player could not continue playing. He complained of generalised pain in the knee, which had swelled up by the time he was helped off the pitch and rested on a bench. He was given ice packs and anti-inflammatories by the first aid team. There was no external injury.
Two hours later, he was able to stand on both feet. Following the conclusion of the game, the team manager insisted that he was taken to hospital because the knee was quite swollen.
No bony injury
At the A&E four hours later, an X-ray was taken and the patient was told he had no bony injury. The examining clinician noted the swelling but reassured him that this would go down with rest, ice, elevation and anti-inflammatories.
The patient was sent home with a support bandage and crutches, which he used for a week as prescribed. By then, the pain and swelling in the knee had decreased.
Following a rest period of two weeks, the patient started training and readied himself for the next match, which was to be in a month. However, during a practice session, his knee suddenly collapsed when he attempted to kick a ball with the opposite foot.
He saw his GP and obtained a referral to a specialist. During the time he was waiting to see the specialist, he attempted to play football, but was unable to do so, because he had several episodes of the knee giving way, which he described as: 'The knee felt very unsteady and would not support my body weight.'
The last of these episodes was particularly painful and he noticed his knee was quite swollen the next morning. He did not attempt to participate in any sport, for fear of damaging his knee further.
Examination by the specialist and a subsequent MRI confirmed ACL rupture with a medial meniscal tear. The patient eventually underwent arthroscopic ACL reconstruction and partial meniscectomy.
History is essential
This case shows the importance of the history. A significant mechanism of injury must not be overlooked. There should be a specialist referral or a review following initial assessment.
Owing to the delayed diagnosis, this patient continued contact sport with an ACL-deficient knee, resulting in a further meniscal injury.
Section 6: Evidence base
MRI showing a tear in the lateral meniscus (small purple areas, left) (Photograph: SPL)
There is an extensive amount of literature available and many websites providing information about various aspects of knee injuries.
Patients often research the topic for themselves and may present with a multitude of questions.
- Olsen OE, Myklebust G, Engebretsen L. Exercises to prevent lower limb injuries in youth sports: cluster randomized controlled trial. BMJ 2005; 330: 449.
- MacMahon PJ, Palmer WE. A biomechanical approach to MRI of acute knee injuries. AJR Am J Roentgenol 2011; 197(3): 568-77.
- Poolman RW, Aboulai JAK, Conter HJ et al. Overlapping systematic reviews of ACL reconstruction comparing hamstring autograft with bone patellar tendon bone autograft: why are they different?
J Bone Joint Surg Am 2007; 89: 1542-52.
- This topic falls under statement 15.9 of the GP curriculum, 'Rheumatology and musculoskeletal medicine'.
|CPD IMPACT: EARN MORE CREDITS|
These further action points may allow you to earn more credits by increasing the time spent and the impact achieved.
1. Gage BE, McIlvain NM, Collins CL et al. Epidemiology of 6.6 million knee injuries presenting to United States emergency departments from 1999 through 2008. Acad Emerg Med 2012; 19(4): 378-85.
2. Milewski MD, Sanders TG, Miller MD. MRI-arthroscopy correlation: the knee. Instr Course Lect 2012; 61: 525-37.
3. Giuliani JR, Burns TC, Svoboda SJ et al. Treatment of meniscal injuries in young athletes. J Knee Surg 2011; 24(2): 93-100.
4. Hart AJ, Buscombe J, Malone A, Dowd GS. Assessment of osteoarthritis after reconstruction of the anterior cruciate ligament: a study using single-photon emission computed tomography at ten years.
J Bone Joint Surg Br 2005; 87(11): 1483-7.