Tuberculosis of knee joint is third common osteoarticular tuberculosis after spine and hip.
Knee tuberculosis or commonly called TB knee accounts for nearly 10 percent of all skeletal tuberculosis.
Relevant Anatomy of Knee Joint
The knee joint is the largest joint in the body. It is a superficial joint and because of its large size and shape, not as stable as the hip joint which is a deep joint. For stability, it depends upon the strength of the capsule, the collateral, and cruciate ligaments and the surrounding muscles.
The knee joint is reinforced by the oblique popliteal ligament posteriorly, collateral ligaments medially and laterally. Anterior and posterior cruciate ligaments provide stability to femur and tibia during movements of flexion and extension.
Anteriorly quadriceps expansion, patella, the ligamentum patellae (infrapatellar tendon), and aponeuroses from the vasti medials and lateralis act to stabilize the knee.
The synovial membrane lines the inner aspect of the capsule and extends upwards as the suprapatellar pouch on the anterior aspect of the femur under cover of the quadriceps expansion up to the extent of a hands breadth.
A diverticulum of synovium is prolonged posteriorly and distally between the proximal part of popliteus and the underlying femur and tibia.
Another synovial pouch communicates with a bursa between the semimembranosus and the medial head of the gastrocnemius.
Thus synovial diseases can extend to the synovial prolongations and present as popliteal cysts or swellings.
Pathophysiology of Tuberculosis of Knee Joint
The bacilli reach knee by hematogenous dissemination [ through blood].
The initial focus may start in the synovium, or in the subchondral bone. The bone may be either of tibia, femur or patella.
Sometimes the lesion may be in the juxta-articular osseous region. Once the disease reaches synovium tubercular synovitis occurs which leads to congestion of the synovial membrane. The membrane becomes swollen and laden with tubercles which appears pinkish gray.
Synovial hypertrophy and thickening occur due to granulation tissue. There is an increase in joint fluid secretion. The joint fluid varies from serous, opalescent to turbid yellow. The fluid may contain fibrinous flakes.
As the disease advances, cartilage and bones are involved and tuberculous granulation tissue or pannus erodes the cartilage and causes bone destruction. The disease involves ligaments and other tissues surrounding the joint as well.
The pannus normally starts at the site of synovial reflections and capsular attachments. Sometimes the pannus may grow between cartilage and bone after eroding the cartilage and causes detachment of the cartilage.
Sequestered flakes of cartilage may be found in the joint cavity.
Cartilage loses a glistening appearance and contains fibrillation at the surface. It appears becomes roughened, pitted and softened.
Erosion of the cartilage exposes the subchondral bone. If the disease advances further, there is bony destruction and the whole joint gets filled with granulation or fibrous tissue. The ligaments get disrupted and knee joint goes into triple deformity which consists of flexion of knee joint, posterior subluxation lateral subluxation and lateral rotation, and abduction of the tibia.
A lesion that starts in the bone can lead to tuberculous abscess bone which may be in the epiphyseal or metaphyseal region.
Clinical Features of Tuberculosis of Knee Joint
The patient generally presents with swelling and pain of the knee, insidious in onset and progressing gradually.
Depending upon the site of presentation, the swelling generally shows up as filled parapatellar fossa, suprapatellar pouch, and even popliteal fossa.
Advanced cases may present with swelling and knee deformities.
On examination, the swelling is warm and signs suggestive of knee effusion [patellar tap] may be present.
On palpation, the swollen and thickened synovium gives a boggy or doughy feel and can be rolled between the fingers and the underlying femur.
It is more easily palpated on the medial side of the knee [Because vastus medialis part of quadriceps remains muscular up to its insertion and gets wasted early whereas muscles on the lateral side are aponeurotic and covered by thick iliotibial band].
The skin may lose wrinkles and become edematous.
The site of involvement would be tender, most marked at the synovial reflections and along the joint line.
Early in the stage, there is a terminal restriction of movements [synovitis stage]. In the stage of arthritis, movements get restricted increasingly and become painful as well.
There is a wasting of the quadriceps muscle, especially the medial part.
Spasm of muscles and contracture of hamstrings particularly the biceps femoris leads to a deformity of the leg in flexion, posterolateral subluxation, external rotation, and abduction. This is also called triple deformity.
Tensor fasciae latae muscle action through the iliotibial band further increases the deformity. In long-standing cases, posterior capsule of the knee joint gets contracted.
Imaging of Knee Tuberculosis
X-ray in synovitis stage may be absolutely normal or may show generalized osteoporosis and increased soft tissue swelling.
[A translucent lesion may be visible in case of articular and bony focus.]
X-rays in arthritis stage reveal loss of definition of articular surfaces, marginal erosions. The joint space is reduced and articular bones are destroyed.
In advanced stages, there is a marked loss of joint space, joint destruction, and deformation. Osteolytic cavities and tubercular sequestra [dead bone]may be visible. Triple deformity if present can be visualized.
MRI would show reveals soft tissue involvement in a better way. MRI is useful in cases where soft tissues are involved but bone is not involved and x-rays do not show any abnormality.
With increasing health awareness, the patients now present early and MRI has got an increasing role in the diagnosis of various conditions.
MRI is able to reveal minimal joint fluid in TB knee and helps to reach at early diagnosis.
Lab Studies of TB Knee
Routine Lab Tests
Complete blood count, ESR, and CRP would show inflammation though the levels may not correlate with the severity of the disease.
Joint Fluid Study
The joint fluid is aspirated under aseptic conditions and is subjected to biochemical, cytological and microbiological studies.
Fluid can be subjected to studies like PCR and culture for mycobacteria.
A synovial biopsy may be taken in cases where there is difficulty in diagnosis to help differentiate from other diseases.
Differential Diagnosis of Tuberculosis of Knee Joint
Treatment of Tuberculosis of Knee
Antitubercular drugs are the mainstay of the treatment. Skeletal traction is added to prevent or correct flexion and subluxation deformity and to keep the joint surfaces distracted.
After the disease has responded and quieting acute local signs, gentle active and assisted knee bending exercises are begun.
After about three months, ambulation with suitable splints and crutches is permitted which should be continued for about 6-10 months at least.
Corrective traction and corrective plaster are needed in some cases with deformity.
In the stage of synovitis, if the disease is not responding favorably, arthrotomy and synovectomy should be done. These procedures are also undertaken if the diagnosis is uncertain
Synovectomy can be accompanied by the removal of loose/rice bodies, debris, pannus, and loose articular cartilage.
If there are juxta-articular foci, the curettage of the involved bone is done.
Drug treatment and traction and ambulations are continued in the postoperative period.
In patients with advanced arthritis, the knee joint may be considered for fusion. Arthrodesis is considered in painful fibrous ankylosis cases too.
Arthrodesis provides a painless stable knee at the cost of motion. It is advisable to apply a light above-knee cast to simulate motion restriction preoperatively simulating the functional restrictions imposed by arthrodesis of the knee joint. A light above the knee plaster cast is applied to the limb leaving the ankle and foot free for walking. The patient is sent back to his/her social domestic and professional environments. The patient and his family are advised to report back in the hospital after 3 to 6 weeks, with their mind made up regarding the acceptability of operation.
An osteotomy of the lower femur may be needed in cases where the knee is not in an acceptable position or there is a varus or valgus deformity is present.
As with hip arthroplasty, there are encouraging reports on knee arthroplasty in healed as well as active tuberculosis. But more studies are awaited before the final word could be out on this.
Whether healed or active, the surgery of arthroplasty is always done under the cover of antitubercular drugs.
Prognosis of Tuberculosis of Knee
The most important determinant of recovery is the stage of involvement at the commencement of antitubercular drugs. Patients who begin treatment at the synovitis stage have an excellent prognosis with complete healing and an excellent range of movements.
In advanced cases where arthritis has set in, there would be a loss of a range of movements. The severity of loss depends on the stage of arthritis.
The patient may continue to have a painless and fairly mobile joint for many years in many cases.
Arthrodesis results in loss of movements at the knee but is a feasible choice for many. With increasing encouragement from the success of arthroplasty in knee TB, the future may provide a disease-free mobile knee joint to these patients.