Posterior Knee Dislocation Following Septic Arthritis of the Knee

Abstract

This is a report of a case of nontraumatic posterior knee dislocation following septic arthritis in a 59-year-old male with leucopenia secondary to sarcoidosis. After confirmation of the diagnosis with synovial fluid analysis, the patient was taken emergently to the operating room for arthroscopic-assisted irrigation and debridement. Arthroscopy demonstrated intact cruciate ligaments but confirmed the diagnosis of septic arthritis demonstrating purulent synovial fluid. He was discharged after multiple repeat irrigation and debridement procedures and intravenous antibiotics improved his symptoms. The infection appeared to resolve as his laboratory values normalized following treatment. Three months later, however, x-rays revealed a nontraumatic posterior knee dislocation. We hypothesize that the soft tissues including the cruciate and collateral ligaments were made incompetent by enzymatic digestion and stretching with mobilization which allowed the knee to dislocate without a traumatic event.

Introduction

This case report describes a nontraumatic posterior knee dislocation following septic arthritis. To date there have been no reports of such a case in the English literature. Knee dislocations are most common in the anterior direction and associated with a high-energy trauma which further highlights the uniqueness of this case. We have obtained written informed consent from the patient to publish his case for print and electronic publication and the identity of the patient has been protected.

Case

A 59-year-old male presented to the emergency department with the acute onset of right knee pain, effusion, and 102°F fever. Physical examination also revealed limited knee range of motion, tenderness to palpation along the joint line, and pretibial edema with open lesions on the left leg. There was a history of 2 corticosteroid and 3 Hyalgan injections into the knee for osteoarthritis the last of which was 10 months previously. He had longstanding history of sarcoidosis, leucopenia, anemia, and congestive heart failure. Laboratory values at presentation revealed a white blood cell count of 6.2 thousand cells/μL, an erythrocyte sedimentation rate of 88 mm/s, and a C-reactive protein of 5.6 mg/L.

The patient’s knee x-rays revealed severe, bilateral tricompartmental osteoarthritis and joint aspiration revealed 2 mL of turbid material a white blood cell count (WBC) of 25 000 WBC/mm³. The gram stain demonstrated gram-positive cocci. A magnetic resonance imaging of the knee was obtained while waiting for the gram stain and revealed a large effusion, cartilage loss, intact cruciate ligaments, and increased signal suggesting edema in the distal femur and proximal tibia.

With the diagnosis of septic arthritis, he was taken to the operating room for arthroscopic-assisted irrigation and debridement (I&D) within 12 hours of his initial presentation. Intraoperatively, there was noted to be thickened yellow synovial fluid and the cruciate ligaments were intact. Synovial debridement was performed using an anteromedial working portal. There was no evidence of sarcoidosis in the knee and no tissue sample was sent for pathologic evaluation.

Cultures ultimately grew methicillin-sensitive Staphylococcus aureus and postoperatively he was treated with intravenous nafcillin based on the sensitivities obtained. His physical examination continued to demonstrate a knee effusion, tenderness to palpation, and development of a flexion contracture without any varus or valgus deformity. Based on the continued pain and physical examination findings, the patient returned to surgery for one more arthroscopic and 2 open I&Ds. Six weeks later, the patient’s antibiotic regimen was switched from nafcillin to linezolid due to nafcillin-induced acute interstitial nephritis. Three months later, it appeared that his infection had been successfully eradicated with normalization of his erythrocyte sedimentation rate to 22 millimeters per second and C-reactive protein to 0.6 milligrams per liter. The knee had a range of motion from 5° to 100° of flexion throughout his postoperative course, and the patient was noted to walk with an antalgic gait but up to this point no ligamentous laxity had been noted on physical examination. However, follow-up x-rays at this 3-month postoperative visit revealed that he had sustained a nontraumatic posterior knee dislocation (Figure 2). The knee is at present irreducible via closed methods, and the patient is currently awaiting open reduction and knee arthrodesis.

Figure 2.

Lateral radiograph of the right knee demonstrating posterior knee dislocation with periosteal bone formation along the anterior femoral cortex.

Figure 1.

A, Anteroposterior radiograph of the right knee at initial presentation; B, lateral radiograph of the right knee at initial presentation.

Discussion

Knee dislocation following septic arthritis is likely an extremely rare occurrence as we could find no reported cases in the literature. There have been reports of other joint dislocations following infection. Bagheri et al¹ reported on a pathologic shoulder dislocation secondary to septic arthritis from intravenous drug abuse. Intravenous drug abusers are at great risk of septic arthritis of unusual joints with unusual organisms.¹ Fortunately, our patient had no history of intravenous drug use.

Chu et al² reported on posterior dislocation of a cruciate-retaining total knee arthroplasty following acute bacterial infection. The mechanism of dislocation was loosening of the femoral component and tear of the posterior cruciate ligament near its femoral insertion. The tear occurred when the patient attempted full weight bearing following treatment of the infection.

Gompels and Darlington³ reported on septic arthritis in a patient with rheumatoid arthritis causing bilateral shoulder dislocation, and Walker and Rang⁴ reported on radial head dislocation following septic arthritis of the elbow.

Risk factors for septic arthritis have been shown to include advanced age, immune deficiency, recent surgery, and the presence of implants.⁵ It has been determined that corticosteroid injection, as this patient had undergone, carries an infection rate⁶ of 0.1%. This patient may have been inoculated with his injections and placed at further risk of bacterial proliferation due to his immunocompromised state secondary to sarcoidosis. Once the joint is inoculated, bacterial virulence factors and the immune response incite inflammatory changes to the surround tissues.⁶ This includes the recruitment of polymorphonuclear leukocytes which release proteolytic enzymes that are destructive to both the pathogen and the host tissue.⁶ Staphylococcus aureus in particular has also been shown to release lysosomal enzymes to surrounding tissues within hours of inoculation, hastening destruction of cartilage and surrounding tissue.⁶

Following inoculation, 4 stages of joint infection were described by Gäechter from observations during athroscopic treatment.⁷ First opacity of the synovial fluid develops along with redness and possible petechial bleeding of the synovial membrane.⁷ In the second stage, severe inflammation may be observed with gross purulence and fibrinous deposition.⁷ The third stage is characterized by thickening of the synovial membrane and the formation of compartments giving the arthroscopic view a “sponge-like” appearance.⁷ In the fourth and final stage, there is aggressive pannus formation with infiltration of the cartilage as well as possible undermining. Only in this fourth stage do radiologic signs such as subchondral osteolysis, erosions, and cysts appear.⁷

The etiology of this patient’s septic knee was likely related to his immunocompromised state secondary to sarcoidosis, leucopenia, and congestive failure. Pretibial edema with open lesions on the left leg may have served as the portal of entry for his pyarthrosis or they may have been fistulous sequelae of his infected joint. The decision for initial approach using arthroscopic techniques was made due to the fact that it had been an effective treatment option in previous cases at this institution and has also shown to be a preferred treatment option in the orthopedic literature.^6,7 The exact mechanism of a knee dislocation following septic arthritis of the knee is unknown. However, we hypothesize that the soft tissues including the cruciate and collateral ligaments were made incompetent by digestion by enzymes from both the pathogenic organism and from the host immune cells and subsequent stretching with mobilization allowed the knee to dislocate without a traumatic event.

The ultimate management of septic arthritis of the knee poses a difficult problem with possible surgical options including knee fusion, resection arthroplasty and cement spacer, and/or total knee arthroplasty (TKA). Total knee arthroplasty poses quite a risky treatment because of the difficulty eradicating infection once implants have been placed in the knee joint. Farrell and Bryan⁸ reported on TKA performed as salvage procedure in patients with irreversible knee destruction secondary to bacterial arthritis. The average knee flexion in these cases was 85° postoperatively and there were no clinical signs of infection. The authors noted the inherent risk of chronic prosthetic infection and other complications. In this patient with risk factors for immune compromise and overlying chronic skin wounds, it was decided that elective arthrodesis was preferable in the face of his high risk of an infected total knee prosthesis.

Conclusion

We report a case of a nontraumatic posterior knee dislocation following septic arthritis. Orthopedic surgeons should be aware of this potential but rare complication whenever treating septic arthritis of the knee.