Complete ejection of a ring sequestrum 8 years after external fixation of the tibia

Abstract

On the day of scheduled debridement for a persistent pin tract infection, a 23-year old man presented himself carrying a small bony ring sequestrum that had spontaneously ejected from his tibial wound 1 week earlier. Eight years prior to presentation, he was treated for an open crural fracture which was stabilised with an external fixator. Revision of the operation notes revealed that the placement of this external fixator was performed without predrilling.

Background

Although pin tract infection is a known complication of external fixation, with an incidence of up to 32%, the development of a fracture-related infection (FRI), possibly leading to sequestra, is rare and occurs in only 4% of patients.^1–7 Besides individual patient factors such as advanced age, the presence of medical comorbidity and smoking, technical factors are also proven to contribute to the development of an FRI after external fixation. Stability of the external fixator, the technique used for pin insertion and postoperative pin site care are all known factors that contribute to the development of an FRI.^{8 9}

Insertion of pins (Schanz screws) without predrilling has shown to result in excessive heat generation, which can subsequently lead to thermal necrosis of the adjacent bony tissue. This increases the risk of the formation of a (ring) sequestrum, but can be prevented by predrilling the pin sites with a sharp, water-cooled drill at a low rotational speed prior to manual insertion of the pins.^{8 9}

We present a case of spontaneous ejection of a ring sequestrum from a tibial fistula 8 years after external fixation of the tibia. The aim of this report is to create awareness towards the fact that the technique used during pin insertion of an external fixator deserves special attention in order to avoid thermal necrosis. This reduces the risk of sequester formation and subsequent development of an FRI.

Case presentation

Eight years ago, a 23-year-old man was treated with an external fixator for a Gustilo grade II open crural fracture which he had sustained after a motor vehicle accident. Revision of the operating notes revealed that predrilling was not performed prior to placement of the unicortical pins. The external fixator was removed 4 weeks after application, as it had been chosen to be the definitive treatment of the fracture.

After the removal of the external fixator, the tibial fracture healed adequately. However, purulent discharge from the proximal tibial wound was seen intermittently for the next 8 years, despite several attempts of both antibiotic and surgical treatment. The patient was referred to our tertiary referral centre with a chronic pin tract infection of the left proximal tibia (figure 1A) and surgical treatment was scheduled.

Figure 1.

(A) Photograph of left proximal tibia at presentation with chronic fistula. (B) Close-up photograph of the tibial wound, showing the spontaneous ejection of the sequestrum. (C) Photograph of ring sequestrum which was ejected from the tibial wound a week prior to the planned surgery.

To our surprise, the patient presented himself on the day of planned intervention carrying a small ring sequestrum, that had spontaneously ejected from his tibial wound (figure 1B), with a diameter of approximately 12 mm (figure 1C).

Investigations

Prior to the spontaneous ejection of the sequestrum, radiographs of the left tibia showed a radiolucent aspect of the bone adjacent to the pin tract, along with a calcification of 8 mm in the soft tissue anterior of this area (figure 2A). MRI and CT revealed a small cavity with a fistula in the proximal tibia and a small bony fragment in the adjacent soft tissues, possibly implicating the presence of a second small sequestrum (figure 2B, C). Laboratory investigations showed raised inflammatory markers, with a C-reactive protein of 150 mg/L and a leucocyte count of 12.6×10⁹/L.

Figure 2.

(A) X-ray of the left tibia showing a radiolucency along the pin tract and a small calcification in the soft tissue anterior of the tibia. (B) CT and (C) MRI scan of the left tibia. The axial plane shows the presence of a small cavity in the bone and a fistula through soft tissue. A small bony fragment (smaller than the ring sequestrum visualised in figure 1B is present in the soft tissues. (D) Preoperative X-ray of the tibia showing the absence of the calcification that had previously been seen in the soft tissue anterior of the pin tract.

Preoperative radiographs performed after ejection of the sequestrum confirmed the absence of the calcification that had previously been seen in the soft tissue anterior of the pin tract (figure 2D).

Treatment

Following adequate disinfection, a tourniquet was applied to minimise the bleeding and improve surgical visibility. An oval incision was made in the longitudinal direction of the fistula tract. Subsequently, drill holes were made to create an oval-shaped bony hinged cover, measuring 1×2 cm, around the fistula tract. Surgical debridement of the bony cavity was performed and six deep-tissue cultures were obtained from the tibial cavity using a ‘no-touch technique’.¹⁰ Besides some yellow-coloured translucent spongious tissue, the cavity contained no evident pus and the overlying cortex looked viable and intact. After thorough cleansing, the cavity was filled with an absorbable, gentamicin-loaded, calcium sulfate/hydroxyapatite biocomposite (figure 3). Primary closure of the soft tissues was achieved tension free. Empirical vancomycin was administered by intravenous infusion according to our local protocol and a pressure bandage was prescribed for 72 hours postoperatively.

Figure 3.

Postoperative X-ray of the left tibia, showing filling of the cavity with gentamicin-loaded, calcium sulfate/ hydroxyapatite biocomposite.

Outcome and follow-up

Deep-tissue cultures grew Staphylococcus aureus for which the patient received antibiotic treatment (initially empirical intravenous vancomycin for 5 days until the identification of the pathogen and the antibiogram were known, followed by oral clindamycin for the duration of in total 6 weeks). Postoperative imaging showed the extent and adequate filling of the cavity with absorbable, gentamicin-loaded, calcium–sulfate/hydroxyapatite biocomposite after debridement (figure 3). Despite some initial serous wound leakage which did not require additional treatment, wound healing progressed without any further complications. At 12-month follow-up, there were no signs of recurrence and the patient was asymptomatic and fully mobilising.

Discussion

Spontaneous ejection of a ring sequestrum following pin tract infection is a rare occurrence. In the past 20 years, this has only been described in two case reports.^{11 12} Differently from our report however, these cases described only radiological signs of a ring sequestrum and not its complete ejection from the wound.

Walter and Cramer reported the spontaneous ejection of a ring sequestrum in 10-year-old male patient 2 years after external fixation for a femoral fracture.¹¹ Unlike our case however, this patient presented himself with local and systemic signs of infection. Surgical debridement was necessary to remove the ring sequestrum.

Vaishya and Vaish reported a case of ring sequestrum of the tibia in a 43-year-old man, where the bony sequestrum was extracted by saucerisation.¹² Similarly to our case, this resulted in a swift wound healing within 2 weeks’ time.

It is mainly the lower extremities that are prone to the development of pin tract infections after external fixation, with the tibia and femur being the most commonly affected.¹³ After short-term fixator use, S. aureus is the most frequently found pathogen in bacterial wound culture. Gram-negative bacteria, such as S. epidermidis and Escherichia coli, typically cause infections after long-term external fixator placement.^{3 12}

Besides individual patient factors, technical factors are proven to strongly contribute to the development of an FRI.^{8 9} Technical advances such as hydroxyapatite coating of the external fixation pins for improved bone–pin interface strength have also been introduced to reduce the risk of developing pin tract infections.¹⁴ Other developments include titanium pins with antibiotic coating or antimicrobial coated polyurethane sleeves for external fixation pins.^{1 15}

Management of FRI mainly includes surgical and antimicrobial treatment.¹³ The first step comprehends extensive surgical debridement, that should not only be focused on removal of sequestra and clearing out of the cavity, but also on debridement of local soft tissue. Another important aspect of the surgical procedure is obtaining a minimum of five uncontaminated deep-tissue cultures.¹⁰ The remaining cavity can be filled with an absorbable antibiotic-loaded bone substitute as part of the dead space management, followed by primary closure or reconstruction of adjacent soft tissues.¹⁶

Postoperatively, after having obtained tissue samples, treatment consists of adequate antimicrobial therapy. It is recommended to start empiric intravenous broad-spectrum antibiotic therapy, including a lipo/glycopeptide and an agent against gram-negative bacilli. Factors such as local epidemiology of antibiotic resistance rates, antibiotic formularies and individual risk factors should play a role in the choice of empirical therapy. Once the definitive culture results become available, therapy should be adapted accordingly.¹⁷

Pin tract infections can cause long-term morbidity and are often an underestimated yet possible preventable complication of external fixation. One of the essential steps is to use a careful surgical technique to reduce thermal necrosis of the surrounding osseous and soft tissues as much as possible. We present a case involving a patient who experienced the spontaneous ejection of a ring sequestrum that contributed to his chronic pin tract infection of the tibia. After meticulous surgical and antibiotic treatment, he remained symptom free at 12-month follow-up.

Learning points.

• Predrilling with cooling prior to pin insertion lowers the amount of thermal necrosis, thus reducing the risk of a chronic pin tract infection.

• When suspecting a chronic pin tract infection, an MRI scan can help to visualise the fistula tract, the possible cavity and the sequestrum.

• An aggressive approach, consisting of obtaining deep uncontaminated tissue cultures, extensive surgical debridement, dead space management, tension-free closure of the soft tissues and adequate antibiotic treatment, is needed to successfully treat chronic pin tract infections.