Prosthetic joint infection (PJI) complicates 12% of primary total knee arthroplasties but represents one of the most devastating complications with significant morbidity ^[1]. Chronic PJI with multiple draining sinuses indicates established biofilm formation and extensive soft-tissue compromise ^[2]. The presence of intramedullary infection extending to diaphyseal levels represents an uncommon variant requiring specialised management approaches. Morbid obesity with a body mass index (BMI) of ≥40 kg/m² significantly compounds PJI management complexity through impaired immune response, compromised wound healing, and increased mechanical stresses ^[3]. Two-stage revision remains the gold standard for chronic PJI management with infection eradication rates of 78–85%, though success rates decrease significantly in morbidly obese patients ^[4]. This case presents extensive intramedullary infection, innovative elastic titanium pin spacer fabrication, and a successful staged surgical approach following multiple failed interventions.

Patient demographics and history:
A 60-year-old morbidly obese male (weight 110 kg, BMI 44 kg/m²) presented to our tertiary centre with a chronic right knee infection following primary total knee arthroplasty (TKA) performed 14 months previously. The patient had undergone multiple debridement procedures at the index surgery centre, including an unusual transverse medial incision approach, yet infection remained uncontrolled.

Clinical presentation:
The patient presented with complete inability to bear weight, severe persistent pain, and multiple purulent draining sinuses extending proximally to the tibial level. Physical examination revealed extensive wound dehiscence, multiple active sinus tracts with copious pus discharge, surrounding tissue induration, and complete loss of knee function (Fig. 1). The patient was afebrile but showed signs of chronic illness.

Investigations:
Laboratory investigations demonstrated elevated inflammatory markers: Erythrocyte sedimentation rate (ESR) 94 mm/h, C-reactive protein (CRP) 16.8 mg/L, and leukocytosis 13,200/μL. Multiple pre-operative cultures from sinus drainage showed no bacterial growth. Radiographic evaluation revealed complete loosening of both femoral and tibial components with extensive peri-prosthetic osteolysis (Fig. 2).

First-stage surgical management:

Infected implant removal and debridement:
The patient underwent comprehensive infected implant removal through an extensive approach. During removal, a unique finding was the complete absence of pus at the implant-cement interface and cement-bone interface, contrary to typical PJI presentations. However, extensive intramedullary pus was discovered extending to mid-diaphyseal levels in both femoral and tibial canals, presenting significant challenges for thorough debridement.
Meticulous debridement of all necrotic and fibrous tissue with pus was performed, followed by extensive pulse lavage irrigation. Multiple tissue and bone cultures were obtained but remained negative for bacterial growth.

Innovative spacer fabrication:
An articulated antibiotic cement spacer was fabricated using cement moulds for femoral and tibial components. Uniquely, an elastic titanium pin (Ti-6Al-4V alloy, 2 mm diameter, 200 mm length) was incorporated for the intramedullary cement spacer component in the tibia for enhanced infection control and mechanical stability. The spacer was loaded with vancomycin (4 g), gentamicin (2.4 g), and clindamycin (1.2 g) per 80 g of cement.

Wound management challenges:
The medial transverse wound from previous surgery could not be closed in a tension-free manner, necessitating application of stay sutures with drain placement (Fig. 3). This represented a significant soft-tissue management challenge in the morbidly obese patient with compromised tissue quality.

Post-operative management, first stage:
Empirical intravenous antibiotics (Clindamycin and Piperacillin-Tazobactam) were administered for 8 weeks while monitoring infection control parameters. Serial inflammatory markers showed progressive improvement with CRP decreasing to 4.2 mg/L and ESR to 38 mm/h. Post-operative radiographs demonstrated appropriate spacer positioning with maintenance of joint space and alignment (Fig. 4).

Staged wound closure:

After 8 weeks, on confirming infection control, the medial wound was addressed through a staged approach. Wound margins were refreshed, and fibrous tissue was excised to a bone-deep level, enabling tension-free wound closure.

Second-stage surgical management:

Timing and preparation:
Four weeks following medial wound closure and after confirmation of infection control through normalised inflammatory markers (CRP < 0.5 mg/L, ESR < 25 mm/h), the patient underwent second-stage reimplantation.

Spacer removal challenges:
During second-stage surgery, significant complications were encountered with spacer removal. The femoral spacer component had fragmented into multiple pieces and migrated, while the intramedullary spacer was broken and separated from the tibial base plate spacer near the keel junction. These broken spacer components required specialised removal techniques using osteotomes and careful dissection.

Reimplantation procedure:
All fibrous tissue was meticulously removed, and the dermal layer was mobilised to reduce wound volume, enabling tension-free closure after reimplantation. A progressive constrained kinematics revision TKA with stemmed components was implanted after extensive pulse lavage. Tissue cultures and biopsies were obtained intraoperatively. Immediate post-operative radiographs showed excellent component positioning (Fig. 5).

Post-operative course, second stage:
The patient received empirical intravenous antibiotics for 2 weeks. The perioperative period was complicated by fever and urinary tract infection with Escherichia coli, requiring adjustment of antibiotic therapy based on culture sensitivity reports. Wound healing progressed satisfactorily with suture removal at 2 weeks.

Rehabilitation and outcome:
The patient became symptom-free at 1 month and achieved full weight-bearing after 1 month of physiotherapy. At more than a year’s follow-up, he remains completely symptom-free with the ability to perform activities of daily living independently. Range of motion (ROM) achieved was 0–120°, and radiographs showed stable prosthetic components with excellent bone-implant integration and no evidence of recurrent infection (Fig. 6).

Intramedullary infection presentation: 
The unusual presentation of extensive intramedullary pus extending to mid-diaphyseal levels without involvement of the implant-cement interface represents a rare variant of chronic PJI ^[5]. This finding suggests haematogenous seeding or retrograde infection through medullary canals, requiring specialised debridement techniques to prevent contamination spread.

Innovative spacer fabrication: 
Conventional antibiotic cement spacers have reported failure rates of 10–20% in modern series ^[6]. The incorporation of elastic titanium pins represents a novel approach for managing intramedullary infection ^[7]. This technique provides mechanical reinforcement while maintaining antibiotic delivery capacity. The elastic properties of titanium pins reduce stress concentration and fracture risk compared to rigid reinforcement methods, while their superior magnetic resonance imaging compatibility enables artefact-free imaging for infection monitoring.

Staged wound management approach: 
The innovative staged approach to wound closure in this morbidly obese patient demonstrates the importance of tissue quality optimisation before definitive closure ^[8]. The 8-week interval allowed for infection control and tissue conditioning, enabling tension-free closure and preventing wound dehiscence complications common in obese patients.

Management of culture-negative PJI: 
Despite multiple culture attempts, all specimens remained negative for bacterial growth. This occurs in 7–39% of PJI cases and requires empirical broad-spectrum antibiotic coverage based on clinical presentation ^[9]. The presence of multiple draining sinuses provided a definitive diagnosis according to musculoskeletal infection society criteria.

Literature comparison: 
The success rate achieved in this case aligns with reported two-stage revision outcomes of 78–85% infection eradication, despite the high-risk patient profile and complex presentation ^[10]. The functional outcome (0–120° ROM) compares favourably with literature reports showing significant improvements in knee function following successful two-stage revision.

Limitations: 
This single-case report has inherent limitations, including short-term follow-up (approximately 13 months), inability to provide comparative data with conventional spacers, and need for larger cohort validation of the elastic titanium pin technique. Long-term durability and cost-effectiveness analysis require further investigation and biomechanical studies.

This case demonstrates successful management of an exceptionally complex chronic PJI with intramedullary extension using innovative surgical techniques in a morbidly obese patient. Key success factors included recognition of unusual infection patterns, innovative elastic titanium pin spacer fabrication, a staged wound management approach, comprehensive infection control protocols, and management of spacer-related complications. The case contributes novel insights to orthopaedic literature: the first description of extensive intramedullary PJI management using the elastic titanium pin spacer technique, demonstration of a successful staged wound closure approach in morbidly obese patients, and a comprehensive management protocol for complex PJI cases following multiple failed interventions.

Complex prosthetic joint infections with intramedullary extension can be successfully managed using innovative spacer fabrication techniques and staged surgical approaches, even in morbidly obese patients following multiple failed treatments.