Clinical Benefit of Hydroxyapatite-Coated Versus Uncoated External Fixation: A Systematic Review

Akash Patel, Anik Ghai, Amarjit Anand

Akash Patel, Imperial College Healthcare NHS Trust, The Bays, South Wharf Road, St Mary’s Hospital, London W2 1NY, the United Kingdom
Anik Ghai, Brighton and Sussex University NHS trust, Royal Sussex County Hospital, Eastern Road, BN2 5BE, the United Kingdom
Amarjit Anand, Imperial College Healthcare NHS Trust, The Bays, South Wharf Road, St Mary’s Hospital, London W2 1NY, the United Kingdom
Correspondence to: Anik Ghai, Brighton & Sussex University NHS trust, Royal Sussex County Hospital, Eastern Road, BN2 5BE, the United Kingdom.
Email: dranikghai@doctors.org.uk
Telephone: +07964 585 352
Received: March 3, 2016
Revised: April, 14, 2016
Accepted: April 17, 2016
Published online: June 28, 2016


AIM: Common complications of external fixators include pin-loosening, infection and loss of reduction/malunion. These are increased with prolonged fixation. The main aim of this systematic review was to investigate HA-coated versus uncoated external fixator and determine benefits in terms of pin loosening, infection and loss of reduction/malunion.

MATERIAL AND METHODS: A systematic literature search using PubMed, EMBASE, OVID SP, Cochrane database, ClinicalTrials.gov website and the references of the studies identified was undertaken on 26th August 2014. Comparative trials investigating HA-coated versus uncoated external fixation pins were identified. These were limited using strict eligibility criteria and critically appraised as per the CASP and CONSORT guidelines. Primary outcome measures included pin loosening and infection. Secondary outcome measures included loss of reduction/malunion.

RESULTS: Seven studies were identified of which five studies demonstrated a statistically significant reduction in loosening with HA-coated pins. There was insufficient evidence to demonstrate any clinical benefits, such as numbers needed to treat to avoid early removal of pins. Review of the studies included also demonstrated insufficient evidence to determine any significant clinical benefit with regards to infection and malunion. Critical appraisal demonstrated average methodological quality of the studies.

CONCLUSION: HA-coating of external fixator pins improves bone fixation and reduces loosening in patients undergoing prolonged fixation procedures, such as leg-lengthening, but the influence on infection and malunion is not clear. Further large, well-designed randomised controlled trials with observer blinding, standardized pin insertion and pin care investigating clinically relevant pin loosening, infection and malunion are recommended. Patient reported, functional outcome measures should also be considered.

Key words: External fixation; Hydroxyapatite-coated external fixator pins; External fixtor pins; Leg lengthening

© 2016 The Authors. Published by ACT Publishing Group Ltd.

Patel A, Ghai A, Anand A. Clinical Benefit of Hydroxyapatite-Coated Versus Uncoated External Fixation: A Systematic Review. International Journal of Orthopaedics 2016; 3(3): 581-590 Available from: URL: http://www.ghrnet.org/index.php/ijo/article/view/1627


The external fixator has been used for more than a century[1]. Despite many advances in the development of external fixation techniques, pin- tract infection and loosening are still significant complications. They are the most commonly occurring complications during limb lengthening using external fixation, with some studies reporting an incidence of up to 80%[2-4].

Indications for external fixators include damage control orthopaedics[5] and definitive treatment frames used in managing acute fractures, bone loss/deformity and limb lengthening. The ability of current external fixator designs to allow gradual reduction and realignment with frequent, small adjustments makes them extremely useful for treating these problems. Ilizarov began to develop the basics concepts of definitive external fixator treatment in 1951 after seeing many WWII veterans with tibial fractures. Since then, external fixation techniques have developed considerably and are now essential components in the treatment of congenital and post-traumatic short limbs. Clinical success or failure of limb lengthening is gauged by the amount of length produced, quality of bone generated and avoidance of complications during and after the lengthening process[6].

Loosening and infection are common complications of external fixation. The exact frequency of these problems is difficult to ascertain from the literature as different definitions are used for diagnosis[7]. However, major infections of the pin track, which require intravenous antibiotics and pin removal, have been reported as occurring in 5.8% of cases[7]. Some studies have reported overall rates of infection, including less severe infections, as high as 100%[8]. It is not always clear whether loosening or pin infection occurs first, but there is an association. Pin site infection is also associated with osteomyelitis and loss of reduction. Loosening may result in fluid accumulation around the pin and subsequent increased risk of infection[9].

These complications are especially important to consider for limb lengthening treatments due to the prolonged duration of fixation and high mechanical loads. Diminution of extraction torque with time occurs even in ideal fixation conditions with no infection[10,11].

Pin loosening may occur as a result of thermal and mechanical cortical damage that occurs on pin insertion. Insertion technique and frame stiffness are the most frequently considered of several important factors in pin loosening[12]. The use of tapered self-drilling, self-tapping half pins on power drills with diligent soft tissue care seems to be optimal[1]. Other factors to consider for improving fixation at the pin-bone interface include mechanical pin design and number of pins[13]. Regular pin site cleaning regimes should also be considered to reduce infection. However, there is still no consensus regarding optimal pin care[14].

The use of pin coatings to enhance fixation and reduce infection has been increasingly investigated over the last 10 years[1]. These coatings include hydroxyapatite, titanium and silver. Pins coated with silver have been shown to be associated with less bacterial colonisation than uncoated pins, but their clinical performance has not been found to be definitively superior to that of uncoated pins[1]. Masse et al showed there is a potential for systemic silver absorption with their use and discontinued their study on safety and ethical grounds.

Hydroxyapatite is a crystalline molecule which constitutes 65% of the mineral component of human bone[15]. It has widely and effectively been used in orthopaedic surgery as a bone substitute and prosthetic coating[16] due to its osteoconductive[17] and osteoinductive properties[18]. Studies investigating the use of hydroxyapatite as a coating for external fixator pins have shown promising results in terms of fixation[13,19,20].

It is clear that pin loosening and infection are significant problems associated with external fixators, especially in limb lengthening procedures. Hydroxyapatite coated pins have been previously investigated in animal studies with good results[20]. Therefore, the purpose of this systematic review is to evaluate their use specifically in the treatment of limb lengthening/distraction osteogenesis in humans as well as other procedures involving external fixation therapy including orthopaedic trauma surgery.

The main objectives of this review are to: (1) Conduct a reproducible literature search identifying studies comparing the use of hydroxyapatite coated versus uncoated external fixation pins; (2) Evaluate whether there is any clinical benefit with primary outcome measures being pin infection and pin loosening (extraction torque) and secondary outcome being malunion/loss of reduction; (3) Critically appraise the evidence and describe the findings in light of existing studies.


Literature Search

The Pubmed database was searched on 26th August 2014 using keywords and strict eligibility criteria with no limit regarding the year of publication. The search terms “external fixators” and “hydroxyapatite” were used. The studies identified were further limited by selecting “humans” and “English language articles” only. Each of the remaining studies was carefully analysed. Only comparative (randomized and non-randomized) studies evaluating infection and loosening in hydroxyapatite coated versus uncoated external fixation pins were included. A systematic search of EMBASE using OVID SP, Cochrane database, ClinicalTrials.gov website and the references of the studies identified was also conducted. This did not reveal any additional studies. No statistical analyses were performed due to inhomogeneity between study populations, types of external fixator/pins used and lack of consistency in definition of outcome measures.

Eligibility Criteria

Eligibility criteria for exclusion and inclusion of studies for this systematic review are summarised in Table 1. Below is a breakdown of eligibility criteria for the trials to be included in terms of population (participants), intervention, comparitor and outcomes: (1) Participants: Patients undergoing treatment using external fixation; (2) Intervention: Hydroxyapatite-coated external fixation pins; (3) Comparitor: Uncoated external fixation pins; (4) Outcomes: Primary - infection (pin-site, osteomyelitis), loosening (extraction torque); Secondary-malunion, loss of reduction/bone apposition, radiological (osteolysis), other complications.

Study Selection and Abstraction

Two authors (AP and AG) independently reviewed the abstract of each publication and included or excluded papers. The authors discussed all studies included/excluded. If there were any disagreements or discrepancies in selection, the third author (AA) was consulted. When inclusion or exclusion was not possible based on the abstract, the full text versions were downloaded. In addition, the reference lists of the studies included were reviewed by hand to identify any articles not originally identified during initial electronic search.

After having passed the initial selection process of identifying suitable studies based on inclusion and exclusion criteria, the selected studies were further assessed on methodological quality using the critical appraisal checklist (adapted from Critical Appraisal Skills Programme- CASP, Oxford[21]) for an article on treatment and the revised CONSORT checklist[22] for reporting randomized trials were utilized.

Each study was assessed and scored on three different aspects adapted from the CASP checklist: (1) Whether the results of the trial were valid (Section A); (2) What the results were (section B); and (3) Whether the results would help locally (section C).

Each reviewer scored each study based on answering the 11 different questions from the CASP checklist. These scores were used to guide assessment as to whether the study was to be included or excluded into the review. Where there were discrepancies in scores between different studies, consensus was sought using the third reviewer (AA).

All checklists were used to guide assessment of the studies identified from the literature search. All checklists have limitations; therefore the CONSORT and CASP tools were only used as guidelines.

The next step involved extracting the information from each study. This included the data such as the intervention, comparator, baseline characteristics of each study (e.g., study design, sample size, research size, study period, participant demographics) as well as the primary and secondary outcomes of each study. The data extracted is summarised in Table 2.

Methods used for assessing risk of bias in individual studies

To assess the reliability of results from the studies, investigations into potential sources of bias was carried out, looking at potential sources at both the study and outcome level. Once studies were selected into the review, a key part of the analysis stage was considering the risk of bias in the methodology of each study. The Delphi list[23] was used to help assess the risk of bias for the RCT selected in this review. Points were only awarded when a criterion was clearly satisfied.

Two authors (AP and AG) reviewed each study for its internal validity using the Delphi list and scored each study with a quality score. Any discrepancies amongst scores were reviewed by a third reviewer (AA) and a final decision was made based on the agreement of all reviewers. Other risks of bias also investigated for, included; selection bias, performance bias, attrition bias, detection bias and reporting bias.

The stages adopted in assessing the risk of bias was guided by recommended approaches from the Agency for healthcare research and quality methods guide (AHRQ)[24].


Search Results

The Pubmed search identified a total of 42 articles. Following limitation to “English language articles” and “human species”, 21 studies remained. The abstracts were thoroughly screened and 7 studies comparing hydroxyapatite coated with uncoated external fixation pins were identified. These are summarised in Table 2 and Table 3. The flow diagram in Figure 1 illustrates the search strategy.


The main purpose of this systematic review was to determine whether HA-coating of external fixation pins has a beneficial effect on rates of loosening and pin-track infections compared to uncoated pins. The secondary objective was to determine whether HA-coating influenced malunion and loss of reduction.

Five of the studies in this review found that HA-coating of pins improves bone fixation and therefore reduces loosening in patients undergoing external fixation. However, the results of the studies of Pieske et al[25,26] found that although there was a trend towards a superior clinical outcome, the results showed no significant superiority of HA-coated pins against uncoated pins and that the majority of clinical pin-site parameters were comparable in both groups. At the end of the fixator therapy, Pieske et al[26], found there were a total of 9 loose pins out of 76 in the steel group (11.8%) compared to 6 loose pins out of 76 in the HA-coated group (7.9%), whilst Pieske et al[25], showed there were 10 loose pins in the titanium group (12.5%) compared to 6 loose pins in the HA-coated group (7.9%). Pieske et al[26] found that although there was a lower rate of loose pins in the HA group, this was not significant (P = 0.864) and that the benefit of advanced pin-bone bonding in the HA group was clinically irrelevant. Furthermore the fixation therapy in both of these studies was only for a period of 6 weeks. In other studies which have demonstrated a significant improvement in bone fixation, such as for lower leg lengthening, external fixation treatment time is much longer, extending on average up to 500 days[25]. During long-term fixator treatment, the beneficial effects of HA-coated pins becomes more evident, with improved bone fixation and low rates of pin-loosening seen, compared with the shorter duration of fixation therapy used for this wrist study.

The definitions of pin loosening were extremely variable. Both Moroni et al[13] and Placzek et al[16] did not use a standardized definition of loosening. Placzek et al[16] used the fixation index which they developed to determine screw fixation strength. Pommer et al[28] used both clinical and radiological criteria to define loosening. Piza et al[27] and Caja et al[12] used the same definition of loosening (extraction torque < 150 Nmm per degree). However, as discussed in the critical appraisal, this did not correlate well with clinical results such as the requirement of removing or changing pins. Pieske et al[25,26] defined loose pins as those that were manually extractable with an extraction torque value of grade 4. Despite the difference in definitions used, it is reasonable to say that all of the papers excluding Pieske et al[25,26] show that HA-coating reduces the risks of pin loosening. All of the studies stated that the extraction torque of uncoated pins was lower than HA-coated pins; this was highly statistically significant in all the studies except for Pieske et al[25,26]. However, there is insufficient evidence to comment of clinical benefit such as numbers needed to treat to avoid removal of external fixator pins.

Six of the seven studies included evaluated pin-track infection as a specific outcome measure. The study by Caja et al[12] did not evaluate pin-track infection. Each of these studies used a different definition for infection. Placzek et al[16] used the classification by Schmidt et al (1990) and reported no significant difference in rates of pin-track infection between HA-coated and uncoated pins. Piza et al[27] used Checketts classification (see Appendices) to define pin-track infection and again reported no significant differences in rates of infection between the two groups. Pommer et al[28] used the microbiological criteria of Mahan et al[30] and reported a significantly lower rate of infection in the HA-coated group. Pieske et al[25,26] evaluated infection at each pin-site specifically assessing for signs of erythema surrounding the pin, grading the drainage as well as scoring pain intensity using a numeric rating scale (NRS) (0-10) for each pin-site. In Pieske et al[26], two pin-track infections requiring daily pin-site care and oral antibiotics occurred in the HA group (2.6%) compared with four in the steel group (5.3%) (P = 0.601). These were non-significant results. Therefore, with regards to pin-track infection, there is insufficient evidence to determine whether the use of HA-coated external fixator pins has any significant beneficial effect. The only study to evaluate the effect of HA-coated pins on malunion was undertaken by Caja et al[12]. They concluded HA-coated pins reduced axial deformity, however there was no significant difference between groups in the final bone angle achieved and in the number of patients requiring manipulation. Despite the significant difference in axial deformity, it is not clear whether this has any effect on functional outcome, which is extremely important to consider.

Similar studies investigating the influence of HA-coating of external fixator pins on loosening and pin-track infection have been undertaken in different groups of patients. Moroni et al[31] investigated patients with tibial fractures. The study reported a significantly lower extraction torque than insertion torque for the uncoated groups, whereas the HA-coated group extraction torque was unchanged. Half of the uncoated pins developed infection compared to none of the HA-coated pins. Magyar et al[19] conducted a randomised study investigating patients treated with the hemicallostasis for osteoarthritis of the medial side of the knee. They reported a significantly higher extraction torque (P < 0.05) for the HA-coated pins. Also, 18 of the uncoated screws were loose compared to 1 HA-coated screw. They found no statistically significant difference in pin-site infection.

Overall, with regards to the CASP[21] and CONSORT[22] guidelines, the methodological quality of the studies included in this systematic review was average. The specific deficiencies of each study have been discussed previously in the critical appraisal. In summary, however, these deficiencies include: (1) Poorly defined eligibility criteria; (2) Small sample sizes; (3) No discussion regarding use of prophylactic antibiotics; (4) Poor randomisation techniques (inadequate details of sequence generation, allocation concealment and implementation method); (5) No standardization for pin insertion (depth of threads, number of pins per bone fragment); (6) No power calculations; (7) No confidence intervals; (8) No observer blinding; and (9) No discussion with regards to losses to follow-up.

Other limitations of the studies included in this review include differing surgical techniques and external fixation equipment. This makes comparison of the results of the studies difficult, however it improves the external validity and generalisability of the conclusions of this review as current practices are reflected. Another limitation of this review is the fact that no functional, patient reported outcome measures were considered. Also, none of the studies discussed cost and patient acceptability specifically. These points are important due to the current economic climate and increased, possibly painful, force required to remove HA-coated pins, respectively.

There are other factors which may have affected the findings of this systematic review. Firstly, the selection criteria excluded case reports, non-comparative case-series, editorials, abstracts, and unpublished studies. Although this was undertaken to ensure the studies included were of appropriate high quality and had been through a peer-reviewed process, it may also have introduced publication bias. Secondly, non-English language articles were excluded. This was due to the difficulties with translation. Nonetheless, it may have limited the breadth of literature review. Finally, the search strategy was based on a computer search process. Studies by Colville-Stewart have demonstrated that computer searches may omit some articles, and consequently, limit the scope of this review[32].

In conclusion, despite variability in definitions and methodological deficiencies, all studies except Pieske et al[25,26] demonstrated a significantly reduced risk of loosening in the HA-coated pins. This is especially important to consider in patients undergoing leg lengthening, who require prolonged fixation. However, clinical benefits in terms of pin removal or exchange are not clear. Only Pommer et al[28] discussed pin removal and demonstrated a higher rate in the uncoated group, conferring a possible clinical benefit. With regards to infection, there is insufficient evidence to confidently state any risk reduction with use of HA-coated pins. There is reduced axial deformity with use of HA-coated pins, however Caja et al[12] reported no significant difference in final bone angle or number patients requiring manipulation. Considering the results of this systematic review and previous studies, there is considerable scope for further trials. A well-designed, large randomised controlled trial with observer blinding and standardisation of pin-insertion, pin-care and post-operative rehabilitation is required. A power calculation should be undertaken. The numbers needed to treat with HA-coated pins to decrease the risk of clinically important loosening, infection and deformity should be calculated. Longer follow-up of patients undergoing leg lengthening procedures would also be useful. Patient reported, functional and disease specific outcome measures as well as cost and patient acceptability should also be considered.


The author(s) declare that they have no competing interests.


There has been no funding for this paper.


A.P is the lead author and carried out the majority of the research for this review as well as the interpretation and analysis of the study results. A.G was responsible for reformatting the original paper as well as contributing to the critical appraisal and selection of papers into this review. A.A assisted in developing the eligibility criteria and selecting papers. He also assisted in critical appraisal.


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Peer reviewer: Xianhua Cai, MD, PhD, Professor, Department of Orthopaedics, Wuhan General Hospital,627 Wuluo Road, Wuhan, 430070, China.


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