The influence of hydroxyapatite coating of external fixator pins on pin loosening and pin track infection: A systematic review

Abstract

Objective

The primary objective of this systematic review of published randomised controlled trials was to evaluate whether there was a clinical benefit in terms of pin loosening and pin track infection, or deep infection, associated with hydroxyapatite coating of external fixator pins. The secondary objective was to evaluate whether there was a clinical benefit in terms of loss of alignment or malunion associated with hydroxyapatite coating of external fixator pins.

Methods

Studies included were identified by a PubMed search for relevant randomised controlled trials on the 20th of December 2007. A systematic review was performed.

Results

All of the studies concluded that there was significantly less pin loosening in the HA-coated groups although the definition of loosening was based on different criteria. However, there was insufficient evidence to properly evaluate the clinical benefit in terms of the numbers needed to treat to avoid premature pin removal. There was also insufficient evidence to evaluate whether any clinical benefit is gained by using HA-coated pins with respect to deep infection and malunion.

Conclusion

A well designed large randomised controlled trial is required to determine the numbers needed to treat with HA-coated pins to reduce the incidence of clinically relevant pin loosening, axial deformity and pin track or deep infection.

Introduction

External fixation is a minimally invasive technique that allows fracture reduction by placing a scaffold-like frame outside of the extremity and transfixing the bone using wires and pins. It offers some advantages over internal fixation and/or plaster cast immobilisation in that it can allow continued movement of a joint if it does not span it, it can be applied rapidly without disrupting fracture haematoma and it does not require extensive soft-tissue dissection. These features have made external fixation an essential component of damage control orthopaedics which is an approach to managing the multiply injured patient by stabilising the orthopaedic injuries without worsening the overall physiology by the “second hit” of a major orthopaedic procedure.¹⁷

Other advantages of external fixation include its ability to allow gradual reduction where indicated. This allows transfixed bone to be slowly realigned with frequent small adjustments to the external apparatus making it extremely useful in the management of deformity correction and bone loss.

Pin track infection and pin loosening are frequent complications of external fixation. The clinical frequency of pin track problems is hard to establish because of difficulties in interpreting the literature. This is due to a lack of homogeneity in the definitions for diagnosing pin track infection and the variable frequency of follow up in different studies.¹

Major pin track infection defined as requiring pin removal and administration of intravenous antibiotics, averages 5.8%. Overall rates of pin infection, including less severe infections, have been reported as high as 100%.⁵ It is unclear whether loosening and separation precede or follow the onset of pin sepsis. In either case, infected pin sites are associated with pin loosening, loss of reduction and osteomyelitis. Furthermore the presence of infection can result in high complication rate if secondary intramedullary nailing is carried out—a common scenario in damage control orthopaedics. The incidence of deep infection after conversion of an external fixator to an intramedullary tibial nail is reported as up to 71% when performed after there has been evidence of pin site infection. This is in contrast to an incidence of 0–5.9% in the absence of pin track infection or where it has been adequately treated.⁵

Pin track infection and pin loosening are intimately related but their pathophysiology is not clearly understood. Pettine et al. showed that a progressive reduction in the pin extraction torque occurs with time, even when no infection is present and occurs independent of pin position.¹⁴ This loosening may occur as a result of mechanical or thermal cortical damage that occurs on pin insertion and many attempts have been made to improve the technique. For example, it has been reported, albeit controversially, that the optimal diameter of the pilot hole should be 0.1 mm less than the core diameter of the pin to be inserted as this theoretically allows plastic deformation of bone rather than microfracture which may permit bacterial entry into bone.12, 19 Other methods for improving the fixation at the bone–pin interface have concentrated on increasing the number of pins, increasing the stiffness of the fixation and changing the mechanical design of pins.¹⁰ Despite these technical considerations the bone–pin interface remains the weakest link in the stability of an external fixator and loosening results in fluid accumulation around the pin and subsequently higher risk of pin track infection.⁴ Clearly there are also other factors that may increase the risk of deep infection such as grade of open fractures, general status of the patient, and pin site cleaning regime.

Numerous authors have attempted to improve fixation or reduce infection at the bone–pin interface by coating of external fixator pins. Coatings considered have included titanium, silver and hydroxyapatite (HA). Attempting to manipulate the bone–pin interface in this way is not a new concept and indeed Parkhill reduced pin track infection using silver coated screws over a century ago.¹¹ A more recent in vivo study by Masse et al., has shown that although there was a trend to reduced infection with silver coated pins there was also a significant increase in plasma silver levels and the study was discontinued on ethical and safety grounds.⁷

Hydroxyapatite is widely used in orthopaedic surgery due to its osteoconductive properties.¹⁸ It is a crystalline molecule composed of phosphorus and calcium (Ca₁₀(PO₄)₆(OH)₂) and constitutes approximately 65% of the mineral component of human bone.¹³ Its role as a coating for external fixator pins has been investigated in animal and human studies and these have demonstrated increased extraction torques, greater bone–pin contact and significantly lower frequencies of radiographic pin track rarefaction.1, 5, 6, 8, 10