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 Table of Contents  
Year : 2019  |  Volume : 5  |  Issue : 2  |  Page : 54-61

Monofocal distraction of stiff hypertrophic nonunions. How and why does it work? A systematic review and mechanobiological explanation

Department of Surgical Sciences, Division of Orthopaedics, Faculty of Medicine and Health Sciences, Tygerberg Hospital, Stellenbosch University, Cape Town, South Africa

Date of Submission14-Nov-2019
Date of Decision25-Nov-2019
Date of Acceptance29-Nov-2019
Date of Web Publication19-Dec-2019

Correspondence Address:
Prof. Nando Ferreira
Department of Surgical Sciences, Division of Orthopaedics, Faculty of Medicine and Health Sciences, Tygerberg Hospital, Stellenbosch University, Cape Town 7505
South Africa
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jllr.jllr_19_19

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Stiff nonunions represent a unique subgroup of nonunions which pose distinct management challenges. Distraction of stiff nonunion, although seemingly counterintuitive, has for long been shown to provide predictable results. Potential advantages of such a management protocol include gradual correction of deformity and limb length; noninvasive nature with minimal insult to the local biology, and in some cases, resolution of infection. In the present article, we systematically review publications describing the results of monofocal distraction of stiff nonunions. An overall union rate of >95% without bone grafting was seen in 178 patients across 12 included publications. The theoretical mechanobiological explanation for the success of this management strategy is also explored.

Keywords: Mechanobiology, monofocal distraction, stiff nonunion

How to cite this article:
Tanwar YS, Ferreira N. Monofocal distraction of stiff hypertrophic nonunions. How and why does it work? A systematic review and mechanobiological explanation. J Limb Lengthen Reconstr 2019;5:54-61

How to cite this URL:
Tanwar YS, Ferreira N. Monofocal distraction of stiff hypertrophic nonunions. How and why does it work? A systematic review and mechanobiological explanation. J Limb Lengthen Reconstr [serial online] 2019 [cited 2022 Dec 2];5:54-61. Available from: https://www.jlimblengthrecon.org/text.asp?2019/5/2/54/273575

  Introduction Top

Nonunion of long bones presents a complex challenge owing to the enormous psychological, financial, and social implications. Multiple variables have been implicated in the development of fracture nonunion and this, together with the lack of a universally acceptable definition, classification and treatment strategy have made the management of these common complications challenging.[1],[2] A comprehensive holistic approach to the management of nonunions is of paramount importance. Numerous factors have been implicated to potentially contribute to nonunion and are classically divided into local and systemic factors. A more practical division is based on whether the factors are modifiable or not. In principle, all modifiable risk factors, such as smoking, malnutrition, sepsis, diabetes mellitus, Vitamin D, and the use of NSAIDs, should be optimized before undertaking any surgical intervention.

The most commonly used classification system of nonunions was proposed by Weber and Cech in 1976.[3] They divided nonunions into hypertrophic, oligotrophic, and atrophic nonunions. Hypertrophic nonunions show prolific callus formation; are vascular and have excellent healing potential, given the right mechanical environment. Atrophic nonunions, on the other hand, are characterized by an absence of callus and atrophic bone ends, bone vascularity is deficient, and the bone has poor healing potential. Oligotrophic nonunions share the characteristics of both atrophic and hypertrophic nonunions.

The Paley classification of nonunion was originally meant for tibial nonunions but can be applied to other long bones.[4] While the Weber and Cech classification focus on the biology of bone segments, the Paley classification takes the mechanical environment at the fracture site into account. Nonunion without bone loss (Type A) was divided into mobile or stiff. Mobile nonunions (Type A1) are those that demonstrate little inherent mechanical stability. Radiographically, these usually appear as avascular nonunions, with poor callus formation, minimal surface area of contact. Stiff nonunions (Type A2) usually correspond radiographically to what has been termed as hypertrophic nonunion, with exuberant callous and a large area of bony contact. Stiff nonunion may present with (Type A2.2) or without a fixed deformity (Type A2.1).

Conservative management of an established nonunion is limited to low demand patients with little functional impairment. Multiple surgical options to manage nonunions have been described, but choosing the correct strategy is of utmost importance to effect union in the shortest possible time with the least amount of complications. The choice of optimal procedure depends on the type of nonunion, host factors, and ultimately, surgeon preference.

Conventional thinking is that stiff hypertrophic nonunions need stability only, whereas mobile, atrophic nonunions need stability as well as an augmentation of their biological capacity for healing. Stability in stiff hypertrophic nonunions can be provided by either internal or external fixation. However, one the major drawback of internal fixations is that it frequently requires opening of the fracture site, which can potentially disturb the local biology and vascularity. Stiff nonunions further do not allow acute correction of deformity, so a closed intramedullary nailing is often not possible. Intramedullary nailing can be potentially done without opening up the fracture site only in stiff nonunions without deformity (Type A2.1). However, passing the guidewire for reaming in a closed manner through the sclerosed hypertrophic fracture site can sometimes be impossible. External fixation seems to provide a reliable solution, as it not only provides stability but also makes simultaneous correction of deformity possible, without the need for exposure of the nonunion site.

The terms monofocal, bifocal, and trifocal describe the number of sites in the bone where action is taking place. Bifocal and trifocal reconstructions are frequently used in cases of defect nonunions requiring bone transport. Monofocal distraction osteogenesis of stiff hypertrophic nonunions was first described by Gavril Ilizarov and entails a gradual distraction force being applied across the nonunion with exposure of the site.[5] Its usefulness and efficacy have subsequently been confirmed by multiple authors [Figure 1] and [Figure 2]. We aim to conduct a systematic review of monofocal distraction of hypertrophic stiff nonunions and postulate the theoretical mechanobiological reasons for its effectiveness.
Figure 1: Anteroposterior and lateral radiographs of a hypertrophic distal tibia nonunion with 10 varus malalignment

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Figure 2: Anteroposterior and lateral radiographs showing union after circular external fixator closed monofocal distraction of the nonunion site

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  Materials and Methods Top

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were used in the design and conduct of the present systematic review.

Literature search strategy

A literature search pertaining to the role of distraction of stiff nonunions was conducted for all articles published up until 16 June 2019. The search terms distraction and stiff nonunion were used to search the electronic PUBMED/MEDLINE database. An additional search of reference lists identified potentially relevant studies to be reviewed for relevance.

Eligibility criteria

All English language publications pertaining to distraction of stiff nonunions were included, irrespective of the study design, patient age, involving any part of any bone. Articles were excluded if they pertained to infected nonunions, distraction osteogenesis for bone defects, bone grafting along with distraction osteogenesis, animal studies, or included mixed cohort studies where stiff nonunions were part of a larger group and where extraction of information regarding stiff nonunions was not possible [Figure 3].
Figure 3: Flow diagram showing identification and selection of studies

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Eligibility assessment

All eligible publications were reviewed by the two authors (NF, YST) for inclusion. Any disagreement was resolved by discussion between the two authors.

Data extraction and analysis

Information extracted from included publications consisted of the number and age of patients treated, anatomical location of nonunion, time to union, residual leg length discrepancy (LLD) of >1 cm, existence of any malalignment >5° in any plane, number of patients requiring secondary bone grafting, duration of follow-up, any complications related to treatment, and failure to achieve union.

  Results Top

The initial PubMed search identified five potentially relevant publications. Two studies were excluded for not being relevant one additional study was excluded because of containing a mixed cohort of subjects (only three out of 25 had stiff nonunion) which made data extraction of stiff nonunion subgroup patients impossible.[6],[7],[8] The remaining two publications were retained for analysis.[9],[10]

Additional search for articles from reference lists and other search engines revealed an additional 19 articles. Full-text review of the 21 included articles resulted in 12 publications being retained for final analysis [Table 1].[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20] Excluded publications (n = 9) consisted of a narrative review,[21] mixed cohort series,[4],[5],[22],[23],[24],[25],[26] and two papers that included a period of nonunion compression as part of the treatment strategy.[24],[27]
Table 1: Summary of included articles

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Study characteristics

A total of 12 articles, specifically reporting of the outcomes of monofocal distraction of stiff nonunions, were included for final data extraction.[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20] The results of treatment of a total of 178 patients were assessed. As some patients had bilateral pathology, the number of nonunion cases totaled 183. The age of the patients ranged from 11 to 80 years.

Outcome of treatment

The most common anatomical location for nonunion was the tibia (n = 149, 81%) followed by the femur (n = 28, 15%) and the humerus and radius with three cases each. Hexapods were the most common mode of fixation in 89 cases followed by conventional Ilizarov frames in 88 cases. Monolateral external fixator was used in six cases. Follow-up duration ranged from 6 months to 11 years.

In 172 out of 178 patients (96.6%), fractures healed without any secondary intervention. Seven cases of infected nonunions underwent monofocal distraction. All infected nonunions healed and spontaneous resolution of infection was seen in all except one patient who had a persistent sinus.

Time to union ranged from 7 weeks to 15 months. Only one study reported the use of secondary bone grafting in three cases of persistent nonunion.[12] No other patient in any of the studies required bone grafting. A single patient in one study used a bone stimulator to promote healing.[19] Ferreira et al. reported four refractures after frame removals which were successfully treated with repeat frame application.[9] These were judged to be complications of premature frame removal rather than failure of treatment strategy.

All studies except one used 1 cm as the cutoff for LLD. A LLD of >1 cm was seen in six patients. Mahomed et al. reported eight patients with LLD of >1.5 cm at the end of the treatment.[12] Thus, a total of 14 out of 178 patients (7%) had significant limb length discrepancy after treatment. Residual malalignment of >5° of normal was seen in 14 patients (7%). Nine out of these 14 patients were from one study.[12] Schoenleber et al. reported correction to within 5 degrees of neutral in one plane and to within 10° in other planes in three of their patients.[19]

Pin site infection was the most common complication observed. A total of 52 patients out of 149 had pin-tract infection (34%). Catagni et al. reported pin site infection in 10% of all wire sites but did not report the rate of pin site infection per patient in their series of 19 patients while Saleh and Royston did not report on any complications in their ten patients.[13],[17] Five patients experienced wire breakage, and there were five refractures due to premature frame removal. Other complications seen were two mild ankle equine contractures and one peroneal nerve palsy.

  Discussion Top

Distraction of stiff nonunions was first described by Gavril Ilizarov himself.[5] The success of this strategy was subsequently confirmed by many others, including Catagni et al., Paley, and Rozbruch et al.[13],[22],[28]

Stiff nonunions classically lack mechanical stability which can be provided by either internal or external fixation. Conventional thinking dictates that providing compression across the nonunion site provides the required stability to promote healing. Distraction of these nonunions, on the other hand, seems counterintuitive. This is partly because of conventional teaching of the classical AO principles of internal fixation where interfragmentary compression is often employed. The purpose of this review was to analyze the results of distraction of stiff nonunions and hypothesize the reasons for its success.

Excellent results with success rates of over 95% were seen in our review. Only one study had a significant number of poor results.[12] Out of their 33 cases, 29 united without requiring any secondary intervention, 3 required bone grafting, and 1 case required amputation. In 10 of their 18 cases with LLD, leg lengths were restored to within 1.5 cm of the contralateral side. Out of the eight remaining leg length discrepancies, four were corrected with shoe raises and four were corrected with subsequent treatment which was not specified. This series also reported nine tibias with a residual deformity more than 5° in any plane following treatment. If the results of this study are excluded, then only six patients out of 146 had an LLD of more than 1 cm (4%) and only eight patients out of 146 had malalignment of more than 5° in any plane (5%) at the end of treatment.

Pin site infections were the most common complication, seen in approximately 35% of patients. The majority of these were superficial infections that are easily treated by oral antibiotics and local pin site care. Other reported complications included refracture secondary to early frame removal, wire breakage, ankle equinus contracture, and one peroneal nerve palsy. Ferreira et al. emphasized a learning curve associated with the treatment method and highlighted the challenge to confirm union in certain cases.[9] To avoid the complication of premature frame removal and refracture, they proposed a staged “trial of union” protocol before frame removal. First, the hexapod external fixator was dynamized by releasing all six struts. The site of the uniting fracture was then manually stressed and if this did not cause any pain or deformity, the patient was permitted to bear weight. If the patients were able to walk without pain, they were allowed to return home with a fully dynamized frame and encouraged to mobilize, full weight bearing, for a period of 2 weeks. Repeat radiographs were then compared with those before the trial of union; if no deformity had developed, union was deemed confirmed and the external fixator removed.

To appreciate the theoretically basis for how distraction of stiff non-unions can achieve healing, one has to revisit Perren's interfragmentary strain theory.[29] Strain is defined as the change in length of a material at a given mechanical load. The strain tolerance of a tissue is the maximum deformation it can withstand while still exhibiting normal physiological function. Granulation tissue, as found in the initial fracture gap, for example, can tolerate strain of 100%, while lamellar bone only has a strain tolerance of around 2%. This means that as fracture healing progresses, progressively less strain is tolerated at the fracture gap.

Strain (tissue deformation) is mathematically represented by δL/L where δL represents change in length and is divided by L which represents original length. Strain is, therefore, directly proportional to the amount of movement at the fracture gap and inversely proportional to the width of the fracture gap. Failure of bone healing will occur if there is either too much or too little strain. Too little strain results from either too little movement at the fracture site (decreased δL) as seen in extremely rigid fixation or where large fracture gaps exist (increased L). As some strain is needed to induce callus formation, the result of too little strain is an atrophic nonunion with little or no callus formation. Excessive strain results from either a large δL secondary to inadequate immobilization/fixation or very small L as seen in transverse or oblique fractures where some degree of interfragmentary motion is still experienced. In this scenario, the body's natural response is to expand the callus volume in an attempt to reduce interfragmentary motion at the fracture site (δL) to within tolerable limits for union to occur. If successful, bridging callus can form which further reduces δL with eventual union taking place; if not, a hypertrophic nonunion with abundant callus formation is the result.

The distraction of stiff hypertrophic nonunions has a twofold effect on interfragmentary strain. First, the use of a mechanically competent external fixator reduced the interfragmentary motion (δL), while slight distraction increases the gap between the fracture ends (L). In addition, the correction of mechanical alignment mitigates the effects of any shear forces experienced from malalignment. The net effect of these actions is an overall reduction in strain to within tolerable limits and the creation of a mechanical environment that is favorable for bone formation and resultant healing of the nonunion.

Choosing the optimum treatment strategy remains a critical part in the decision-making around nonunion management. Distraction of mobile/lax nonunions, for example, would not result in bone formation, as these entities result from a low strain environment to start with and would rather benefit from an initial period of compression. During our review, we also encountered articles with ambiguous definitions and indications for distraction osteogenesis. One publication described distraction of fresh fractures, while another described distraction of hypertrophic stiff pseudoarthroses.[18],[30] True pseudoarthrosis, false joint, is not expected to be stiff. Although the terms pseudoarthrosis and nonunion are sometimes used interchangeably, this use is discouraged. True pseudoarthrosis is a special subset of hypertrophic nonunions, which is mobile, has fracture ends that are covered with fibrocartilage, and lined by a synovial-like lining.[31],[32] A monofocal distraction strategy for these mobile pseudoarthroses will fail as manipulation of the mechanobiological environment is insufficient to overcome the longstanding biological adaptations seen in these cases. Patients that present with a true pseudoarthrosis would require resection of the nonunion site followed by the most appropriate bone reconstruction.[33]

  Conclusion Top

Distraction of stiff hypertrophic nonunions is an excellent treatment strategy with proven clinical outcomes and union rates exceeding 95%. Although counterintuitive, the practice has a sound theoretical basis and can be explained on the basis of Perren's interfragmentary strain theory. As external fixation remains the most commonly used method of achieving monofocal nonunion distraction, pitfalls intrinsic to these devices, including pin site infection, are inherent to this treatment strategy. Apart from excellent union rates, other potential advantages of this treatment strategy include correction of limb length discrepancy, minimal disruption of local vascularity, obviating the need of bone grafts, and the ability to allow early weight bearing and immediate functional rehabilitation.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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