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Primary haematogenous osteomyelitis of the patella: a rare cause for anterior knee pain in an adult
  1. R K Kankate,
  2. T P Selvan
  1. Department of Orthopaedics, East Surrey Hospital, Redhill, UK
  1. Dr R K Kankate, House 8, Gill Avenue, Guildford, Surrey GU2 5WW, UK (lrk5{at}hotmail.com or raghu{at}kankate.freeserve.co.uk)

Abstract

Acute osteomyelitis of the patella is a very rare condition, which commonly affects children between the ages of 5–15 years. Primary haematogenous osteomyelitis in an adult usually occurs in patients with associated risk factors like intravenous drug abuse, HIV infection, and trauma. This report discusses a similar condition in a 46 year old women with no associated predisposing risk factors. The rarity of this condition and its atypical presentation should be borne in mind while treating an adult patient with anterior knee pain. Point tenderness over the patella should alert a physician to the possibility of osteomyelitis of the patella. The value of bone scan and computed tomography in the early stages to help diagnose this condition has been stressed. The literature has been reviewed and discussed briefly.

  • haematogenous osteomyelitis
  • patella

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The atypical presentation, clinical signs, various investigative modalities, and the treatment of a case of acute primary haematogenous osteomyelitis involving the patella in a 46 year old women with no predisposing risk factors is discussed.

Case report

A 46 year old white woman was admitted under the physicians with complaints of insidious onset pain and swelling in the right knee and calf. There was no history of trauma. She was afebrile and otherwise healthy. On examination of the right leg, generalised tenderness in the leg was noted. However, specific tenderness was not looked for; 90 degrees of painless range of motion in the knee was present. The routine blood investigations and the radiographs of the knee were normal. As the calf was tender to palpation, a clinical diagnosis of deep venous thrombosis was made, which was excluded by a venogram.

The pain around the knee persisted for six months subsequently, when she was referred to the orthopaedic team. Examination at this stage revealed specific tenderness over the patella and also a knee effusion. A raised erythrocyte sedimentation rate of 24 mm/hour was the only haematological abnormality. Blood culture grewStaphylococcus epidermidis, which was thought to be a skin contaminant. Radiographs at this stage revealed a possibility of a lytic lesion in the right patella (fig 1). The anterior knee pain was further investigated by a technetium bone scan, which showed a hot spot in the region of the right patella. Computed tomograms showed erosion of the patella and the clinical diagnosis of an infective pathology of the right patella was made (fig 2).

Learning points

  • Acute haematogenous osteomyelitis of the patella usually affects children between the ages of 5–15 years. It is, however, exceedingly rare in adults. Five similar cases (excluding this one) in adults have been described in the literature, but all were associated with a predisposing risk factor like HIV infection, intravenous drug abuse, or trauma. Our patient was a healthy adult with no associated predisposing risk factors. We think this is the first report of its kind in the English literature.

  • This condition should be included in the differential diagnosis for anterior knee pain in an adult patient.

  • Awareness of this rare condition, which can occasionally baffle treating physicians and surgeons, is necessary and could help prevent delay in management.

  • Early treatment by surgical debridement and administration of sensitive antibiotics ensures an optimal result and prevents the sequel of septic arthritis.

  • Careful clinical examination is mandatory. Pinpoint tenderness over the patella can raise the index of suspicion for acute haematogenous osteomyelitis of the patella. This is probably the single most useful clinical sign. Raised infective haematological markers like C reactive protein and erythrocyte sedimentation rate would also raise the index of suspicion.

  • Technetium bone scan and computed tomography are useful diagnostic tools and should be used where necessary.

Figure 1

Radiograph showing a possible lytic area in the patella.

Figure 2

Computed tomograms showing erosion of the right patella.

Surgery was offered to the patient both with a view to obtain a histological diagnosis and to offer a cure. Arthroscopy revealed an Outerbridge grade 2 change1 in all the three compartments of the knee. Under the same anaesthetic the patellar erosion was curetted through a midline approach and the tissue sent for histological studies which confirmed the diagnosis of acute pyogenic osteomyelitis. Microbiology culture tests grewStaphylococcus aureus. However, the knee aspirate was sterile. The osteomyelitis was treated with intravenous cefuroxime for one week followed by five weeks of oral cephalosporin. The knee was splinted for six weeks in a cylinder cast. At her latest follow up at 18 months she had a painless range of 120 degrees of range of motion in the knee and radiologically the osteomyelitis had healed.

A search for any other infective focus in the body, which could be responsible for acute osteomyelitis of the patella in this patient, was negative.

Discussion

Isolated lesions of the patella are unusual. The list of pathological entities include trauma, degenerative diseases, primary or metastatic tumours, congenital defects, and cysts and infections.2 Acute pyogenic haematogenous osteomyelitis of the patella is exceedingly rare in adults and in children under the age of 5 years. The rarity of the disease in children under the age of 5 years may be due to its cartilaginous nature and because of the fact that it has very little blood supply. Vascularisation proceeds with ossification reaching a maximum at 12 years and at 15 years the patella is ossified completely. After ossification the vascularity decreases. The incidence of haematogenous osteomyelitis of the patella is believed to correspond directly to the increased vascularity of the patella which is maximum between 5–15 years of age and hence it occurs commonly in this age group.3-6 This could also account for it being rare in adults where the vascularity is reduced. In adults a history of trauma is usually present in most of the cases. It is also often believed to be the sequel of a prepatellar bursitis.5 S aureus is perhaps the most common organism responsible for causing acute osteomyelitis of the patella.7 Tuberculous osteomyelitis is also an entity of which one should be aware, but it often tends to be multifocal.7

We could find five cases of acute pyogenic osteomyelitis of the patella in adults described in the literature, but all had an associated risk factor. The first patient was a 25 year old man who had pseudomonas osteomyelitis of the patella but he was an intravenous drug abuser.8 The second case was in an adult who was suffering from HIV.9 Brodie's abscess of the patella has also been described in a 24 year old man after a history of trauma.10 Pyogenic osteomyelitis has been described by Kochar and Srivastava in a 32 year old who had a history of trauma.7 The fifth case was recognised after a pathological fracture of the patella in a 57 year old patient with multiple myeloma.11

The authors believe that this is the first reported case of primary acute haematogenous osteomyelitis of the patella without any predisposing risk factors. Routine plain radiographic evidence is not always of help because the patella is a sesamoid bone in the tendon of quadriceps femoris and is covered by thin lamina with no real periosteum. Periosteal elevation, which is one of the hallmarks of osteomyelitis, is hence absent in patellar infections.3Pinpoint tenderness over the patella can raise the index of suspicion for acute haematogenous osteomyelitis of the patella. This is probably the single most useful clinical sign. Bone scan is a helpful diagnostic tool to localise the problem initially while computed tomography confirms the diagnosis. Though no associated septic focus was found in this case, a careful search to look for one should be made.

The articular cartilage in an adult is thinner than in the child; thus, articular involvement is more likely in the adult.9Sympathetic effusion is often present and can result in a sterile aspirate as we had in this case. Early diagnosis and treatment is hence essential to prevent the eventual complication of septic arthritis.

The authors feel that osteomyelitis of the patella should be ruled out while treating a common condition like anterior knee pain in an adult for which a certain cause has not been identified. This should be included as one of the rare causes for anterior knee pain in an adult. Careful clinical examination along with a raised erythrocyte sedimentation rate or C reactive protein can alert the treating physician of this rare condition, who can then use computed tomography to confirm the diagnosis. Only awareness of the existence of this condition can raise the index of suspicion in the treating physicians and thus avoid the delay in the diagnosis and management as we had in this patient.

References

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