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Painful knees

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Q1: Describe the radiographic findings

The radiograph (see p 691) shows osteoarthritic changes in both knee joints and hypertrophic non-union of fractures of upper third of tibia.

Q2: What is the diagnosis?

The diagnosis is non-union of bilateral symmetrical stress fractures of the tibia associated with osteoarthritis of the knee.

Q3: Discuss the pathomechanics of this condition

Eccentric loading of a long bone sets up tension stresses on its convex side and compresses the concave side.1 A deformity at the knee joint will lead to abnormal stresses in the tibia. Such altered biomechanics in osteoarthritis with varus deformity is known to produce repetitive abnormal stresses on the proximal tibial metaphysis leading to stress fracture.2 Most of the fractures described in the literature are incomplete, unilateral fractures, which heal with conservative management.2–4 Rarely, they may progress to complete fractures2 or even to non-union.1 Fracture repair is stimulated in these fractures leading to callus formation, but as long as the deformity at the knee and hence the altered biomechanical forces persist, union of the fracture is prevented. Because of the stiffness at the adjacent arthritic joint the proximal lever arm acting on the fracture is effectively lengthened thus increasing the deforming force causing stress at the site of the fracture.5

Q4: Discuss the treatment of this condition

Most of the incomplete stress fractures heal with conservative measures. But, in order to prevent recurrence of fractures due to the persistence of abnormal stresses, deformity correction is necessary. This can be done by a high tibial osteotomy or a total knee replacement arthroplasty. Once the fracture has progressed to the stage of non-union, the treatment becomes difficult and operative intervention becomes necessary at this stage.

The ideal treatment described for such cases is by using a modular total knee arthroplasty that corrects the malalignment at the knee joint and converts the tension stresses across the proximal tibia into compression forces. The use of a modular prosthesis allows the tibial stem to be extended across the fracture that acts as an intramedullary splint to provide stability.5 The advantage of such a procedure is that only a single stage surgery is required for fracture union as well as for prevention of recurrence. Intramedullary nailing can also be used to achieve union in such cases. Deformity correction is required in these cases later to prevent recurrence of stress fracture.

Discussion

Osteoarthritis of the knee is a very common condition. The pain intensity and functional disability may vary from patient to patient. Occasionally, a patient whose symptoms are controlled by conservative measures such as analgesics and physiotherapy will experience a sudden deterioration. This is usually attributed to deterioration of arthropathy, or the development of another lesion such as a tear of a degenerated meniscus, a loose body, or osteonecrosis of the femoral or tibial condyle.3

Stress fractures of upper tibia can be a cause for such an increase in symptoms.2–4 The diagnosis of these fractures is often masked in early stages because of improper radiographs, which focus only on the knee joint and do not show the tibial diaphysis. Hence all patients with such symptoms should be carefully examined to look for the point of tenderness in the tibia and full length radiographs should be taken to visualise the tibial shaft. Moreover, the radiographs may not show significant changes for two to three months after the onset of symptoms, hence a bone scan may be needed for early diagnosis.3

Learning points

  • A stress fracture is usually the result of repeated episodes of unaccustomed activity or abnormal stresses.

  • A sudden deterioration of symptoms in case of osteoarthritis of the knee with deformity can be due to a stress fracture.

  • Bone scan is useful in the early diagnosis of such stress fractures as radiographic changes are not obvious in the early stages.

  • Altered biomechanics in cases of osteoarthritis with deformity produce repetitive abnormal stresses over the proximal tibial metaphysis leading to stress fractures.

  • Persistence of abnormal stresses leads to non-union.

  • Ideal treatment for such cases is by a modular total knee arthroplasty that produces compression forces at the fracture site and corrects the deformity as well leading to fracture union.

Final diagnosis

Non-union of bilateral symmetrical stress fractures of the tibia associated with osteoarthritis of the knee.

References

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