Stress fracture of the distal tibia secondary to severe knee osteoarthritis: a case report

INTRODUCTION

Stress fracture is caused by either abnormal stress placed on a normal bone (fatigue fracture) or normal stress placed on an abnormal bone (insufficiency fracture). Both are due to the exertion of cumulative and repetitive low intensity forces on the bone for a period of time. The tibia is the most common site for a stress fracture, accounting for 50% of such fractures. Stress fractures of the tibia secondary to sports-related activities are relatively common but stress fractures of the distal tibial shaft secondary to osteoarthritis of the knee are rare. Such fractures usually involve the proximal tibia.

CASE REPORT

A 61-year-old heavily built woman with a 10-year history of painful osteoarthritis associated with severe varus deformity of both knees had declined total knee replacement for fear of surgery. She was able to walk but with severely deformed varus knees.

In October 2004, she slipped and twisted her left foot. She complained of pain over the distal left shin that did not resolve after 2 weeks of self-medication, and consulted her family physician who arranged for radiographs to be taken. No abnormalities were revealed and she continued to walk on the left foot. The pain gradually worsened over the next 6 months, and she was eventually confined to a wheelchair. The patient noticed a progressive angulated deformity in the left ankle and repeated radiographs demonstrated a malunited fracture at the distal third of the left tibial and fibular shafts. She was promptly referred to the Singapore General Hospital for further management.

Correction of the malunion using an interlocking nail was performed. The patient was discharged uneventfully, but her knee pain persisted due to severe osteoarthritis. Partial weight-bearing was recommended as the underlying varus deformity of the knee had not been corrected and there was a risk that abnormal stress would break the nail. Full weight-bearing was started once the fracture united. Ten months later, the patient underwent a left total knee replacement with simultaneous removal of the interlocking nail and made an uneventful recovery.

DISCUSSION

Stress fractures occur as a result of repetitive abnormal mechanical loading on the bone. The insults are usually chronic and of low intensity such that the fracture is not acute. Risk factors include repetitive activity (as in sports or marching), abnormal biomechanical forces (such as those that occur in hallux valgus, genu varum or valgus, and limb length discrepancy), and systemic diseases that weaken the bone (rheumatoid arthritis, osteoarthritis, osteoporosis, and renal osteodystrophy).

In our patient, the severe varus deformity of the knee imposed considerable stress on the tibia and shifted the mechanical axis of the left foot to the knee and tibia. Stress fractures have been reported to occur in the tibia, metatarsal shafts, fibula, tarsal navicular, lumbar, humerus, femoral neck, femoral shaft, and pubic ramus, with the tibia being the most common site. The junction of the middle and distal third of the tibial shaft-where the greatest curvature occurs-bears the greatest stress. Studies of the strength distribution in the tibia reveal that fracture is more likely to occur where the curvature is large." According to the mechanism of her fall, our patient sustained a rotational force along the long axis of the left tibia. As long bones do not withstand rotational forces very well, this accident was probably a sentinel event, causing the stress fracture to become symptomatic (the patient had a pre-existing stress fracture in the left distal tibia not visible on radiograph).

Physical signs of a stress fracture include tenderness on palpation and localised pain as a result of compression. In the early stages, plain radiographs may reveal no abnormality,12 as in our patient. If a stress fracture is suspected, a technetium -99m bone scan should be performed because it is more sensitive than a plain radiograph.

There are several treatment options for correction of malunion at the left distal tibial and fibular shafts and treatment of osteoarthritis and severe varus deformity of the left knee. The first option is a single-stage total knee replacement with a long stem extension of the tibial component to bypass the fracture site.13 This would have been feasible in our patient if the fracture had occurred in the proximal shaft,14 but standard modular implants with a stem long enough to bypass a fracture in the distal tibia are not available. The second option is a one-stage total knee replacement with correction of the malunion of the distal tibia. This has the advantage of being one procedure with a single anaesthetic and surgical risk. Nonetheless, the procedure would be more extensive and require separate incisions. In addition, the implant used to stabilise the malunion would have to be a plate as an intramedullary nail would interfere with placement of the tibial component. The dissection at the malunion would be extensive, and the plate would be configured to a 'wave' pattern to accommodate the exuberant callus at the malunion. There is a risk of skin breakdown with such extensive dissection and the need to accommodate a plate with a wave configuration. Wound breakdown would put the total knee arthroplasty at risk of developing infection. The third option is a 2-stage procedure. In stage one, malunion of the distal tibial shaft is corrected using an interlocking nail. The procedure is minimally invasive and dissection around the callus is avoided. The nail is a load-sharing device and the risk of refracture is minimal once the fracture has consolidated. A plate is not used as it must be configured to a wave pattern to accommodate the exuberant callus at the malunion. In stage 2, following union of the tibial fracture, total knee replacement is performed with simultaneous removal of the nail. This is less technically demanding, but requires 2 operations, and leaves the varus knee and abnormal mechanical axis untreated initially, which may lead to implant failure. We decided that this last option was the safest for our patient. Manipulation was performed to correct the malunion as the fracture callus was not consolidated. An interlocking nail was then used to maintain the reduction.

Bone mineral density evaluation was not performed in our patient, thus osteoporosis as a contributory cause of the fracture could not be excluded. Stress fractures in patients with osteoarthritis are rare and usually occur in the proximal tibia. Our patient was unusual as the stress fracture occurred in the distal tibia. A 2-stage procedure using interlocking nailing for the distal tibial fracture, and a total knee replacement for the osteoarthritis and varus deformity was performed because it was the safest option.