|
 
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| REVIEW ARTICLE |
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| Year : 2020 | Volume
: 6
| Issue : 1 | Page : 7-12 |
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The effect of fixation dynamization on fracture healing: A systematic review
Nando Ferreira, Yashwant Singh Tanwar, Marilize Burger
Department of Surgical Sciences, Division of Orthopaedics, Faculty of Medicine and Health Sciences, Tygerberg Hospital, Stellenbosch University, Cape Town, 7505, South Africa
| Date of Submission | 11-Apr-2020 |
| Date of Decision | 18-May-2020 |
| Date of Acceptance | 30-May-2020 |
| Date of Web Publication | 30-Jun-2020 |
Correspondence Address:
Prof. Nando Ferreira
Department of Surgical Sciences, Division of Orthopaedic Surgery, Faculty of Medicine and Health Sciences, Tygerberg Hospital, Stellenbosch University, Cape Town, 7505
South Africa
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/jllr.jllr_11_20
Dynamization of fixation has long been used as a strategy to promote fracture healing. Which clinical scenarios would benefit from dynamization, how and when to introduce it and its effectiveness remains to be elucidated. A systematic review of the literature reporting on the use of dynamization in fracture healing using the Preferred Reporting Items for Systematic Reviews and Meta-Analysis was conducted. The primary outcome was time to union and union rate of fractures across all anatomical sites, including adult and pediatric populations. A total of 19 studies fulfilled the inclusion criteria of which 14 studies evaluated the use of dynamization using intramedullary nails, while five studies evaluated external fixator dynamization. The diversity of dynamization strategies, variation in timing of dynamization and contradictory results precludes definitive conclusions. Further research is needed before recommendations for the use of dynamization to improve fracture healing can be considered.
Keywords: Dynamization, fracture healing, interfragmentary motion
How to cite this article:
Ferreira N, Tanwar YS, Burger M. The effect of fixation dynamization on fracture healing: A systematic review. J Limb Lengthen Reconstr 2020;6:7-12 |
| Introduction | |  |
Fracture healing involves a complex coaction between biology and mechanics where changes in the one directly and indirectly affect the other.[1] The mechanical environment plays a vital role in fracture healing and can be described in terms of interfragmentary motion and relative deformation.[2],[3] The resultant strain is important during the initial stages of fracture healing and determines cellular differentiation and tissue formation.[4]
Various tissues have unique abilities to tolerate deformation and strain. Intact bone has normal strain tolerance of 2% before failure, whereas granulation tissue can tolerate strain of up to 100%.[5],[6],[7] A complex interplay between biology and mechanics attempts to manipulate the strain at a fracture or osteotomy site to allow the bridging callus to form and the fracture to unite. This is achieved through either tissue proliferation and differentiation or bone resorption.[8]
Callus formation requires a small amount of relative deformation and will not take place when the strain is too low.[3],[6],[9] A low-strain environment will be produced if the fixation device is too stiff to allow sufficient interfragmentary motion and delayed healing or an atrophic nonunion will result.[10] If the interfragmentary strain remains excessive, resulting in instability, bony bridging by hard callus will not occur in spite of good callus formation, and a hypertrophic nonunion may develop.[11]
Mechanical stimulation during early fracture healing enhances callus formation.[12],[13],[14] Soft callus formation is stimulated by movement of the fracture fragments in an attempt to bring strain to within tolerable limits for bone formation.[12] This initial soft callus is gradually replaced by hard callus, and eventual bony bridging occurs. The formation of hard callus during the later stages of fracture healing can however be compromised by excessive mechanical stimulation.[15] During these later stages of fracture healing (after 6 weeks), high strain magnitudes are not tolerated as the excessive motion disrupts bridging callus and interfere with fracture healing.[15],[16]
Mechanical stimulation also has a direct effect on the physiology of fracture healing. Ilizarov stated that functional load determines the structure, shape, and volume of any limb. This is due to an increase in local blood flow during functional use that aids in tissue growth.[17] Mechanical stimulation also directly influences bone biology on a cellular level by stimulating the proliferation and differentiation of osteoblasts.[17],[18],[19] Mechanical force application patterns, as well as loading magnitude and frequency, also affect bone healing on a biochemical level.[18] The rates of synthesis and degradation of extracellular matrix components are affected by force application patterns. Loading magnitude affects cell size through increasing amounts of intermediate filaments and glycogen particles, while changes in loading frequency can alter mRNA synthesis of various genes.[18] Aggrecan gene expression is also increased in response to mechanical stimulation and leads to an increased proteoglycan scaffold for type II collagen.[17]
Mechanical stimulation provides further benefits in terms of union site remodeling according to “Wolff's law”. This phenomenon was originally ascribed to piezoelectrical charges that are generated in response to mechanical stresses. Osteoblasts on the compressive side are stimulated by electronegative charges, while osteoclasts are activated by electropositive charges on the tension side.[20],[21] This explanation is likely an oversimplification of a complex mechanism that regulates bone remodeling.[22] The current understanding of bone mechanosensation involves strain-generated potentials to explain how bone is able to respond to mechanical stresses.
Dynamization has been employed as a strategy to manipulate this healing process specifically in the setting of fracture fixation with intramedullary nails and external fixators. Its effectiveness is however controversial, and the limited available research shows contradictory results.[10] The questions that still remain are how to effectively use dynamization and at what stage during fracture healing to introduce it.
We report the results of a systematic review of current evidence regarding the effect of dynamization on fracture healing in humans.
| Methods | | |
A systematic review was undertaken according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis statement on all English language publications on the effect of dynamization on fracture healing in human participants [Supplementary Table 1].[23]{Table 1}
Literature search strategy
The literature search of electronic databases was conducted for articles published up to February 23, 2019, and included Google Scholar and the Medline and Embase databases as well as the central registrar of controlled trials in the Cochrane library. The search terms included Fracture, Healing, Dynamization (Dynamisation), and Dynamize (Dynamise), with the Boolean terms “AND” and “OR.” All abstracts were reviewed, and the full articles were obtained for all potentially relevant articles. During initial screening and identification, article titles and abstracts were reviewed for relevance. Following this, secondary identification and screening process identified potentially relevant studies from the reference lists of full-text articles identified in the initial phase.
Eligibility criteria
Studies on the effect of fixation dynamization on fracture healing were considered eligible. All experimental study designs including randomized and nonrandomized control trials and prospective and retrospective cohort studies were included. Only English language studies involving human subjects were included. Animal studies, case reports, congress proceedings, review articles, hypothesis articles, and any publication that was not subjected to peer review were excluded.
Eligibility assessment
All clinical trials were independently screened during initial screening and identification. During secondary identification, full-text evaluation was undertaken. Any disagreement was resolved by discussion between the authors. Where consensus could not be reached, a third adjudicator acted as an arbitrator.
Data extraction and analysis
Data were collected regarding the technique and timing of fixation dynamization and its effect on the rate and quality of fracture healing. Heterogeneity of the research resulted in the inclusion of human studies involving both external fixation and intramedullary nail fixation.
| Results | | |
The initial search strategy identified 135 publications; eight additional studies were identified from reference lists resulting in a total of 143 publications being screened for inclusion. The initial screening and secondary identification identified 38 potentially relevant publications. Full-text review of these 38 publications resulted in 19 publications being retained for the final review. Excluded publications (n = 19) consisted of case reports, animal studies, hypothesis, technical reports, biomechanical, and review articles and articles that were not peer reviewed or had incomplete information for the analysis [Figure 1].
Study characteristics
The 19 included studies were published between 1992 and 2018, spanning a period of 26 years. Publications were assigned a level of evidence according to Sackett's rules of evidence that rank studies according to the probability of bias.[42] Only two publications provided level I evidence. The remaining publications consisted of four level II, three level III, and ten level IV evidence articles.
Study sample sizes ranged from 15 to 194 and included a total of 1133 patients. Only one publication investigated dynamization in a pediatric population. Five publications investigated the effect of dynamization following external fixation, while the remaining 14 publications investigated dynamization after intramedullary nail fixation. Dynamization protocols showed considerable variation between studies. The introduction of dynamization was achieved through axial and elastic strategies, while the timing of dynamization varied from immediately after the initial surgery, up to eight months following the injury.
Nail dynamization
The majority of studies on human participants investigated passive dynamization following intramedullary nailing and did so for the management of acute fracture, delayed union, and nonunion. Nail dynamization as performed in these cases constitutes axial dynamization as an element of axial collapse or telescoping is introduced that might address fracture gaps that were inadvertently left during the initial fracture fixation surgery.
Four studies compared dynamic fixation to static fixation in human femur and tibia fractures. Khalid et al. compared two groups of patients with femur fractures that were treated with either dynamic fixation or static fixation at the time of surgery. In cases where the initial fracture reduction resulted in a gap of <3 mm, there was no difference in healing rate or time to healing between the groups. Where the fracture gap was larger than 3 mm, dynamic fixation resulted in faster time to union but did not alter the union rate.[35] This was confirmed by Hernández-Vaquero et al. who again found no difference between dynamic and static fixation for tibia fractures in terms of union rates or time to union, but patients with a mean fracture gap of 2.33 mm experienced more biological complications when compared to patients with a mean fracture gap of 1.56 mm.[32]
Only a single study compared the effect of dynamization on delayed fracture healing in a randomized control trial. Basumallick et al. compared 50 patients with femoral shaft fractures that were either treated with static interlocked nails or dynamized between 3 and 6 months following the initial surgery.[28] Dynamization was found to shorten the time to union, but it did not affect the union rate. Perumal et al. reported an 80% union rate when they introduced dynamization after 12 weeks from the initial injury.[39] Contradictory to this, Tigani et al. reported a significantly shorter time to union in their static interlocked group compared to patients who underwent dynamization.[40]
Seven studies investigated the effect of dynamization on delayed fracture healing, and union rates ranging between 42% and 66% were reported using this strategy.[25],[27],[36],[38],[41],[43] Wu et al. reported poor results in treating femur delayed union with nail dynamization and most of their patients still required cancellous bone grafting to achieve union.[27],[43] They concluded that the dynamization of intramedullary nails did not promote healing and suggested to rather opt for cancellous bone grafting for the management of delayed union.[27],[43] Similarly, Singh et al. during the treatment of delayed tibia fractures reported that dynamization was only successful in producing union in 66% of cases. The authors concluded that dynamization should only be considered as a treatment strategy for a delayed union in fractures that were statically locked in distraction.[36] Vicenti et al. found that when dynamization was considered for delayed union or nonunion of femur fractures, this should be done early because 4 of 12 dynamization procedures failed when it was done longer than 9 months after the initial injury.[41] Similarly, Huang et al. showed that dynamization before 10–24 weeks had a better outcome than when dynamization was delayed until after 24 weeks. The authors also found that dynamization with a screw preserved in the dynamic hole provided better results than when all screws were removed from one side of the nail.[33]
Two studies reported on the effect of nail dynamization for nonunion management. Litrenta et al. compared exchange nailing with nail dynamization for the management of tibial nonunions. The authors demonstrated high union rates for both interventions but found that comminuted fractures and fractures that were initially fixed in distraction favored exchange nailing over dynamization.[37] Papakostidis however cautioned against dynamization in unstable and atrophic femoral nonunions as they experienced significant complications including significant shortening and rotational malalignment.[31]
External fixation dynamization
As with the dynamization of interlocking nails, external fixation devices with “built-in” dynamization modules allow telescoping and collapse at the fracture or osteotomy site. Almost all the included publications reporting on the dynamization of external fixators used an early axial dynamization strategy for the management of acute tibia fractures.
Singh et al. introduced axial dynamization between 4 and 6 weeks after fixation and reported 88% union in 68 patients.[40] Barquet et al. compared static fixation with axial dynamization, 2–10 weeks from injury, and reported faster healing times in dynamized fractures.[24] Similarly, Foxworthy et al. compared axial dynamization before and after 4 weeks from injury in adult tibias and observed significantly faster healing times in patients dynamized before 4 weeks.[26]
Domb et al. in the only randomized control trial compared the results of 53 pediatric femur fractures treated with external fixation.[29] One group was kept static for the duration of treatment, while the other was allowed axial dynamization at 50 days after injury. They found no statistically significant difference in terms of callus formation, time to radiological union, or time to fixator removal or time to full weight-bearing.[29]
Only one publication employed reverse dynamization as a treatment strategy. Howard et al. used an elastic dynamization external fixator model to produce a rigid configuration after an initial 2–4 weeks period of macromovement. The authors concluded that their results supported the hypothesis that early macromovement followed by fixator rigidity was superior to conventional dynamization regimes.[34]
| Discussion | | |
The aim of this systematic review was to investigate the current evidence regarding the effect of dynamization on fracture healing. Excessive heterogeneity of included studies, specifically related to definitions, experimental design, and outcomes, makes interpretation of these findings challenging.
Roux, in 1881, introduced the concept of “developmental mechanics.”[44] He postulated that multiple cell lines are present in tissues, like callus, and that the mechanical environment provides a survival advantage of one tissue type over another. Pauwels in 1960 related mechanical forces to the differentiation of mesenchymal progenitor cells into either chondroblastic of osteoblastic cell lines.[19] These initial ideas gave rise to our current understanding of mechanobiology and how the mechanical environment influences bone biology.
The mechanical environment has different effects on different stages of fracture healing. Controlled, cyclical interfragmentary micromotion during the initial stages of fracture healing produces callus and early fracture bridging.[3] Excessive motion during the latter stages, however, has a detrimental effect on the union as it prolongs the chondral phase of fracture healing.[45] The net effect is that of a steady increase in fracture stiffness/decrease in motion over the first couple of weeks after an injury.[8]
The term “dynamization” refers to multiple strategies that require definition. Conceptually, dynamization refers to altering fixation stiffness to allow more interfragmentary motion at the fracture or osteotomy site. However, the lack of a universally accepted definition of dynamization leads to a wide variation in the interpretation and application of the technique. Claes et al. however emphasizes an important conceptual distinction between axial dynamization and true/elastic dynamization.[46] They expressed concern that many dynamization experimental models were essentially flawed as the dynamization process also allowed a degree of axial collapse or closure of the fracture gap or osteotomy site and did not only alter the rigidity of the system.[10] This was termed “axial dynamization” and is seen where a fixator incorporates a telescoping mechanism or a nail interlocking screw is removed. Elastic dynamization on the other hand, allows temporary fracture deformation under physiological loading, and recovery of the original fracture dimensions during unloading.[46]
In most cases where interlocking nails are dynamized, only axial dynamization is introduced. The place for this form of dynamization is probably the scenario where initial fixation inadvertently left a critical-sized gap. This was elegantly illustrated by Khalid et al. who showed that dynamization was only effective in producing union in cases where an initial fracture gap of larger than 3 mm existed.[35] This method for management of delayed union and nonunion of fractures is still being advocated by many authors despite a success rates of only around 50%.[25],[27],[36],[38],[41],[43],[47],[48] This is mostly because of the low cost and low morbidity associated with this strategy when compared to bone grafting, exchange nailing, and compression plating.[38],[48] When this strategy is employed, however, care should be taken as axial dynamization can result in excessive collapse and a resultant leg length discrepancy. Axial dynamization should also be reserved for diaphyseal fractures, as metaphyseal fractures have the potential problem of developing angular deformities when exposed to unstable fixation.
Elastic dynamization is easier achieved with the use of external fixators through the exchange of fixator components or staged deconstruction. Although the evidence is conflicting and difficult to interpret, it appears that early elastic dynamization is detrimental to fracture healing.[10] Late dynamization, after bony bridging, may accelerate bone remodeling.[46],[49] The problem with these statements is that they refer to temporal variations relative to the stage of fracture healing and not specific time in days, weeks, or month. As most research is being conducted on animal models and fracture healing times may foreseeably be different between species, the exact quoted times to dynamization or union might not be directly extrapolated to human fracture management.
The concept of “reverse dynamization” was introduced by Glatt et al. in 2012 and described a strategy in which initial low fixation stiffness is gradually increased during treatment.[50],[51],[52] Reverse dynamization as a management strategy affords some theoretical advantages.[5],[52] Low initial fixation stiffness allows greater interfragmentary motion which induces larger callus volume while a gradual increase in stiffness allows bony bridging to occur with less cartilage formation.[50],[51],[53] Other authors like Epari et al. in 2013 also theorized that the optimum fracture fixation would entail flexible fixation during the early phases of healing and rigid fixation during the later stages.[5] Exactly how and at what point during the healing process this management strategy is implemented remains to be established. This strategy could potentially also be combined with late elastic dynamization to aid with callus remodeling once bony bridging has occurred.[46],[49] For obvious reasons, with current internal fixation technology, reverse elastic dynamization is only possible with external fixator devices.
At present, all we can state with any degree of certainty is that the mechanical environment unequivocally contributes to the type and speed of fracture healing. It is possible to manipulate both these aspects of fracture healing, but the most appropriate and effective way to achieve this is yet to be defined. In other words, we can show that dynamization is effective to modulate fracture healing, but exactly how and when to dynamize or if we should use reverse dynamization is still unclear.
The quality of the current review is limited by the caliber of the included studies. The diversity of dynamization strategies, variation in timing of dynamization and contradictory results precludes definitive conclusions. Further research is needed before recommendations for the use of dynamization to improve fracture healing can be considered. In additiona, the standardization of (i) definitions, (ii) timing of interventions, and (iii) reporting of results would greatly benefit the global orthopedic community to assess the value of dynamization.
| Conclusion | | |
The exact role of dynamization in fracture healing remains to be established. As our understanding of mechanobiology and fracture healing evolves, the use and usefulness of dynamization will hopefully become elucidated. Where dynamization appears to be most effective is where a fracture gap of larger than 3mm was left after initial intramedullary fixation, and early dynamization allows for axial collapse and closure of this gap. More research on the clinical application of reverse dynamization is needed.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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