Axial Dynamization Research Articles

Impact of lateral cortical notching on biomechanical performance in cephalomedullary nailing for unstable pertrochanteric fractures.

In pertrochanteric femur fractures the risk for fracture healing complications increases with the complexity of the fracture. In addition to dynamization along the lag screw, successful fracture healing may also be facilitated by further dynamization along the shaft axis. The aim of this study was to investigate the mechanical stability of additional axial notch dynamization compared to the standard treatment in an unstable pertrochanteric femur fracture treated with cephalomedullary nailing. In 14 human cadaver femora, an unstable pertrochanteric fracture was stabilized with a cephalomedullary nail. Additional axial notch dynamization was enabled in half of the samples and compared against the standard treatment (n = 7). Interfragmentary motion, axial construct stiffness and load to failure were investigated in a stepwise increasing cyclic load protocol. Mean load to failure (1414 ± 234N vs. 1428 ± 149N, p = 0.89) and mean cycles to failure (197,129 ± 45,087 vs. 191,708 ± 30,490, p = 0.81) were equivalent for axial notch dynamization and standard treatment, respectively. Initial construct stiffness was comparable for both groups (axial notch dynamization 684 [593-775] N/mm, standard treatment 618 [497-740] N/mm, p = 0.44). In six out of seven specimens the additional axial dynamization facilitated interfragmentary compression, while maintaining its mechanical stability. After initial settling of the constructs, there were no statistically significant differences between the groups for either subsidence or rotation of the femoral head fragment (p ≤ 0.30). Axial notch dynamization provided equivalent mechanical stability compared to standard treatment in an unstable pertrochanteric fracture. Whether the interfragmentary compression generated by axial notch dynamization will promote fracture healing through improved fracture reduction needs to be evaluated clinically.

Mechanical stimulation of distraction regenerate. Mini-review of current concepts

Introduction One of the key limitations of distraction osteogenesis (DO) is the absence or delayed formation of a callus in the distraction gap, which can ultimately prolong the duration of treatment.Purpose Multiple modalities of distraction regenerate (DR) stimulation are reviewed, with a focus on modulation of the mechanical environment required for DR formation and maturation.Methods Preparing the review, the scientific platforms such as PubMed, Scopus, ResearchGate, RSCI were used for information searching. Search words or word combinations were mechanical bone union stimulation; axial dynamization, distraction regenerate.Results Recent advances in mechanobiology prove the effectiveness of axial loading and mechanical stimulation during fracture healing. Further investigation is still required to develop the proper protocols and applications for invasive and non-invasive stimulation of the DR. Understanding the role of dynamization as a mechanical stimulation method is impossible without a consensus on the use of the terms and protocols involved.Discussion We propose to define Axial Dynamization as the ability to provide axial load at the bone regeneration site with minimal translation and bending strain. Axial Dynamization works and is most likely achieved through multiple mechanisms: direct stimulation of the tissues by axial cyclic strain and elimination of translation forces at the DR site by reducing the effects of the cantilever bending of the pins.Conclusion Axial Dynamization, along with other non-invasive methods of mechanical DR stimulation, should become a default component of limb-lengthening protocols.

Should nails be locked dynamically or statically in atypical femoral fractures? – A radiological analysis of time to union and reoperations in 236 displaced fractures with 4 years average follow-up

IntroductionAtypical femoral fractures (AFFs) are associated with delayed union and higher reoperation rates. Axial dynamization of intramedullary nails is hypothesized to reduce time-to-union (TTU) and fixation failure as compared to static locking. MethodsConsecutive acutely displaced AFFs fixed with long intramedullary nails across five centres between 2006 and 2021 with a minimum postoperative follow-up of three months were retrospectively reviewed. The primary outcome was TTU, compared between AFFs treated with dynamically or statically locked intramedullary nails. Fracture union was defined as a modified Radiographic Union Score for Tibial fractures score of 13 or greater. Secondary outcomes involved revision surgery and treatment failure, defined as non-union beyond 18 months or revision internal fixation for mechanical reasons. ResultsA total of 236 AFFs (127 dynamically locked and 109 statically locked) were analysed with good interobserver reliability of fracture union assessment (intraclass correlation coefficient = 0.89; 95% CI = 0.82–0.98). AFFs treated with dynamized nails had significantly shorter median TTU (10.1 months; 95% CI = 9.24–10.96 vs 13.0 months; 95% CI = 10.60–15.40) (log-rank test, p = 0.019). Multivariate Cox regression revealed that dynamic locking was independently associated with greater likelihood of fracture union within 24 months (p = 0.009). Reoperations were less frequent in the dynamic locking group (18.9% vs 28.4%), although the difference was not statistically significant (p = 0.084). Static locking was an independent risk factor for reoperation (p = 0.049), as were varus reduction and lack of teriparatide use within three months of surgery. Static locking also demonstrated a higher frequency of treatment failure (39.4% vs 22.8%, p = 0.006) and was an independent predictor of treatment failure in logistic regression (p = 0.018). Other factors associated with treatment failure included varus reduction and open reduction. ConclusionsDynamic locking of intramedullary nails in AFFs is associated with faster time to union, lower rate of non-union, and fewer treatment failures.

Current trends in the development of long tubular bones osteosynthesis

We reviewed scientific literature on the problem of osteosynthesis of long tubular human bones, published during the last 10 years. The Scopus, Web of Scince, Pubmed, RSCI databases were searched for the articles reporting the results of clinical studies and biomechanical experiments using plate osteosynthesis. The advantages and disadvantages of minimally invasive plate osteosynthesis for different segments have been revealed. The articles reported a lower probability of displacement development in minimally invasive plate osteosynthesis in comparison with intramedullary osteosynthesis, good biological conditions for fracture healing, decreased rate of complications of postoperative wounds due to reduced incisions.
 In the concept of biological osteosynthesis, the advantage of axial dynamization and fracture micro-mobility over absolute rigidity was noted. The study also revealed the influence of the parameters of a plate and osteosynthesis technique on the rigidity of the plate-bone system, such as: the working length of the plate, the number of screws on the plate, types of screws (cortical or locking), the plate material and its profile.
 The bone osteosynthesis seemed to have new directions of evolution. These include far cortical locking screws allowing micromobility under the plate, providing a "controlled dynamization". An experimental technology of Active Locking Plates has been reported, where the screws with angular stability are locked in holes on elastic sliding elements providing micromobility of the screw relative to the plate.
 In general, all the visible results differed in various studies and, sometimes, contradicted each other.

Histomorphometric Analysis of Callus Formation Stimulated by Axial Dynamisation in a Standardised Ovine Osteotomy Model.

The cyclic axial dynamisation of a stabilised fracture is intended to promote callus formation and bone healing. Most studies focused on biomechanical properties or the quantity of new bone formation. Far less is known about the quality of newly formed callus tissues, such as tissue distribution and arrangement within the callus. The aim of this current study was to investigate the effect of cyclic, axial dynamisation on the quantity and quality of callus in an established delayed fracture healing model. In 41 sheep transverse osteotomies with a gap size of 3 mm were stabilised with a unilateral external fixator. In 32 of these, fracture ends were axially stimulated with displacement amplitudes of 0.8 mm, 0.4 mm, 0.2 mm, or 0.0 mm, respectively, for six weeks. In the remaining 9 sheep of the control group, an additional external fixator was mounted to achieve almost total rigidity. Animal material originating from a past animal experiment was reanalysed in this study. Histological thin-ground sections were histomorphometrically analysed regarding the histological structure and composition of the defect region. A slight tendency towards an increase in size of total callus area, area of new bone (nB.Ar), and cartilage (Cg.Ar) was detected with increasing displacement amplitudes compared to the control group. At the anterior callus side nB.Ar and Cg.Ar were significantly larger than at the posterior side in all groups independent of treatment. Regarding the quality of callus, areas of very compact bone were predominant in the treatment groups whereas in the control group a slight shift to more porous bone was observed. No difference of callus compactness was observed between the anterior and the posterior side. The established method to assess the local compactness of callus areas is a useful tool to quantitatively determine the spatial distribution of new bone tissue within the callus. The application of this method in combination with biomechanical testing might reveal interesting relations between tissue distribution and bone strength that, with traditional histomorphometry, cannot be identified.

From Bench to Bedside: How Stiff is Too Stiff? Far-cortical Locking or Dynamic Locked Plating May Obviate the Question.

From Bench to Bedside: How Stiff is Too Stiff? Far-cortical Locking or Dynamic Locked Plating May Obviate the Question.

Dynamic Stabilization with Active Locking Plates Delivers Faster, Stronger, and More Symmetric Fracture-Healing.

Axial dynamization of fractures can promote healing, and overly stiff fixation can suppress healing. A novel technology, termed active plating, provides controlled axial dynamization by the elastic suspension of locking holes within the plate. This prospective, controlled animal study evaluated the effect of active plates on fracture-healing in an established ovine osteotomy model. We hypothesized that symmetric axial dynamization with active plates stimulates circumferential callus and delivers faster and stronger healing relative to standard locking plates. Twelve sheep were randomly assigned to receive a standard locking plate or an active locking plate for stabilization of a 3-mm tibial osteotomy gap. The only difference between plates was that locking holes of active plates were elastically suspended, allowing up to 1.5 mm of axial motion at the fracture. Fracture-healing was analyzed weekly on radiographs. After sacrifice at nine weeks postoperatively, callus volume and distribution were assessed by computed tomography. Finally, to determine their strength, healed tibiae and contralateral tibiae were tested in torsion until failure. At each follow-up, the active locking plate group had more callus (p < 0.001) than the standard locking plate group. At postoperative week 6, all active locking plate group specimens had bridging callus at the three visible cortices. In standard locking plate group specimens, only 50% of these cortices had bridged. Computed tomography demonstrated that all active locking plate group specimens and one of the six standard locking plate group specimens had developed circumferential callus. Torsion tests after plate removal demonstrated that active locking plate group specimens recovered 81% of their native strength and were 399% stronger than standard locking plate group specimens (p < 0.001), which had recovered only 17% of their native strength. All active locking plate group specimens failed by spiral fracture outside the callus zone, but standard locking plate group specimens fractured through the osteotomy gap. Symmetric axial dynamization with active locking plates stimulates circumferential callus and yields faster and stronger healing than standard locking plates. The stimulatory effect of controlled motion on fracture-healing by active locking plates has the potential to reduce healing complications and to shorten the time to return to function.

Dynamic locking plates provide symmetric axial dynamization to stimulate fracture healing.

Axial dynamization of an osteosynthesis construct can promote fracture healing. This biomechanical study evaluated a novel dynamic locking plate that derives symmetric axial dynamization by elastic suspension of locking holes within the plate. Standard locked and dynamic plating constructs were tested in a diaphyseal bridge-plating model of the femoral diaphysis to determine the amount and symmetry of interfragmentary motion under axial loading, and to assess construct stiffness under axial loading, torsion, and bending. Subsequently, constructs were loaded until failure to determine construct strength and failure modes. Finally, strength tests were repeated in osteoporotic bone surrogates. One body-weight axial loading of standard locked constructs produced asymmetric interfragmentary motion that was over three times smaller at the near cortex (0.1 ± 0.01 mm) than at the far cortex (0.32 ± 0.02 mm). Compared to standard locked constructs, dynamic plating constructs enhanced motion by 0.32 mm at the near cortex and by 0.33 mm at the far cortex and yielded a 77% lower axial stiffness (p < 0.001). Dynamic plating constructs were at least as strong as standard locked constructs under all test conditions. In conclusion, dynamic locking plates symmetrically enhance interfragmentary motion, deliver controlled axial dynamization, and are at least comparable in strength to standard locked constructs. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1218-1225, 2015.

Treatment of stable and unstable intertrochanteric fractures with selfdynamisable internal fixator (concept of double dynamisation).

BACGROUND/AIM. Intertrochanteric fractures of the femur are the third most common fractures among all bone fractures. Today in everyday orthopedic practice a number of different methods of treatment of trochanteric fractures of the femur are applied. Despite the improvement in the development of new implants, the percentage of serious complications of the treatment of these fractures remains very high, varying from 10% to 20%. One of the most serious complications of internal fixation of intertrochanteric fractures is nonunion of fractures due to the lack of additional axial dynamisation of implants. The aim of this study was to determine the efficacy of double dynamisation in stable and unstable intertrochanteric fractures treatment using the self dynamisable internal fixator. During the period from 2000 to 2009 we analyzed the use of selfdynamisable internal fixator (SIF implant) in the treatment of 247 patients with stable and unstable intertrochanteric fractures. Fracture types were classified according to the AO Fracture Classification/Orthopaedic Trauma Association Scheme. Salvati and Wilson scoring systems were used for functional assessment considering pain, walking ability and hip movements of operated patients. Of the total number of treated patients, 134 were males and 113 females, aged 19 to 90 (average 49.6) years. More than a half of the patients were older than 50 years. Monitoring of the patients after the operation was carried out clinically and radiographically for a period of three to six months in all the patients, whereas a 2-year follow-up was conducted in 176 (71.2%) patients. The average duration of surgery was 47 min, the average blood loss 145 mL, and the average fluoroscopy time was 16 sec (8-97 sec). The average time for union was 3.7 months (3-6.6 months). Double dynamisation (dynamisation along the neck and shaft of the femur) was observed in 85 (34.4%) patients, and was on average 4.3 mm (1.5-8 mm). All fractures managed with dynamisation implants healed completely within no later than six months after the surgery. In 17 cases there was a cut-out phenomenon of implant, while in seven cases there was mechanical implant failure. Complications were detected within 3 to 6 weeks after surgery, and treated by the method of intramedullary fixation. During the study, there were no cases of infection and thromboembolic complications detected. The concept of double dynamisation improves the fracture healing in the stable and unstable intertrochangeric fractures using the selfdynamisable internal fixator. This biological method of fixation provides healing of intertrochanteric fracture in the optimum period of time, significantly reducing the risk for mechanical failure.

Results of the femur fractures treated with the new selfdynamisable internal fixator (SIF)

PurposeAs axial dynamisation is a recognised method, many authors using interlocking femoral nail perform an additional small operation two months after the primary operation in order to remove one screw so as to provide axial dynamisation. According to the literature, dynamisation happens in about 15–25% of cases, but it cannot be predicted which patient or fracture will need dynamisation. The aim of this study is to present a new selfdynamisable implant and a minimally invasive method for the internal fixation of different femoral fractures.Materials and methodsThe study was conducted between 2000 and 2008 and included 849 patients with 871 fractures receiving the selfdynamisable internal fixator (SIF) for proximal, diaphyseal and distal femur fractures.ResultsThe average operative time was 44 min (23–119 min) and the average fluoroscopy time was 12 s (6–92 s), while the average blood loss was 90 ml (60–250 ml) when a minimally invasive technique was used. None of the patients developed complications during the intra-operative period. Complete follow-up was available in 726 patients with 738 fractures. The healing time was 3.9 months (3–9 months). Healing was achieved in 99.1% of patients. Superficial infection developed in seven fixations (0.9%), while deep infection developed in four patients (0.5%). Screw-breaking occurred within 6–18 weeks in 19 fixations (2.6%). Cut-out phenomenon happened in 24 cases. Spontaneous axial dynamisation was observed in 71 (23.8%) out of 738 fractures, being 5 mm on average (2–12 mm).ConclusionThe SIF is an effective method for the treatment of femoral fractures. This method is particularly valuable in the treatment of comminuted fractures with regard to minimally invasive surgery.Electronic supplementary materialThe online version of this article (doi:10.1007/s00068-011-0157-7) contains supplementary material, which is available to authorized users.

Kinematic adjustability of unilateral external fixators for fracture reduction and alignment of axial dynamization

Bone fracture reduction and bone axial dynamization are important operations which effectiveness can be further enhanced by the use of a unilateral external fixator. By design, axial dynamization can be performed through reciprocating one of its translational joints. However, non-axial dynamization may occur after correcting residual fracture deformity. To explore and to maximize its full potential, the joint adjustment constraint equations for fracture reduction and alignment of axial dynamization under unilateral external fixation are derived. Their physical implications and criteria on the kinematic structure of a fixator are then established. In order to correctly make the alignment of axial dynamization with the proper fracture reduction, this study shows that the linkage of a bone–fixator system should have a minimum of eight degrees-of-freedom (DOFs) with at least two nonparallel rotational DOFs adjacent to both ends of the designated single translational DOF for axial dynamization. Thus, the adjustment of the connection between bone pin/pin clamp of Othofix ® fixator is required, while the alignment of one of the translational joints of Dynafix ® fixator with its bone segment axis during fracture stabilization procedure is a crucial step. A conceptual fixator that requires neither an adjustment of the pin/pin clamp connection nor special pre-alignment is demonstrated. Based on the constraint equations and criteria developed throughout this study, the creation of an effective frame design of external fixation device becomes feasible.

Axial shortening during distraction osteogenesis leads to enhanced bone formation in a rabbit model through the HIF-1α/vascular endothelial growth factor system

Axial micromotion of bone fragments enhances callus formation during fracture repair or limb lengthening. To examine this, we used an axial-shortening model of the tibial callus in rabbits and performed histological analyses. After 10-mm lengthening of the left tibia with an external fixator, we shortened the callus by 2 mm. Radiographs and quantitative evaluation of corrected bone mineral density showed a significant increase in mineralization in the shortened callus (57.3 vs. 36.2%, p = 0.001). Histologically, greater osteoblast proliferation and more vigorous trabecular bone formation were noted in the shortened calluses than in the controls. Around the front of membranous bone formation in the shortened callus, there was a significant decrease in mean percentage area of vascular lumens (1.8 vs. 4.5%, p = 0.009), which seemed attributable to compressive force, and a significantly increased production of vascular endothelial growth factor (VEGF; 422.5 vs. 142.7 pg/mg protein, p = 0.007) and its receptors. There were also increased numbers of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts and proliferating cell nuclear antigen (PCNA)-positive cells. A marked increase of hypoxia inducible factor-1alpha (HIF-1alpha) expression in osteoblasts was also observed in this area. Thus, enhancement of membranous bone formation by static compression or axial dynamization may be at least partly attributable to HIF-1alpha-mediated VEGF induction following the local hypoxia caused by collapse of vascular lumens.

In vivo axial dynamization of canine tibial fractures using the Securos external skeletal fixation system

Bilateral transverse mid-shaft tibial osteotomies, with a 4-mm gap, were performed in purpose-bred research dogs and stabilized using a Securos Type 2 external skeletal fixotor (ESF). Full (100%) axial dynamization of one randomly selected ESF in each dog was performed at 31 days postoperatively. Caudo-cranial radiographs were obtained at weekly intervals, which were qualitatively and quantitatively evaluated (densitometry and ImageJ analysis). The dogs were euthanatized 13 weeks postoperatively, at which time dual energy x-ray absorptiometry (DEXA), peripheral quantitative computed tomography (pQCT), mechanical testing in torsion, and qualitative histological analysis were performed. A two-tailed paired Student's t-test was performed for statistical analysis of all parameters of interest, with significance set at p < 0.05. Three of five dynamized bones bridged quicker, and four of five dynamized bones appeared to have greater callus formation, however, statistically significant differences could not be definitively determined. Statistically significant differences were not found with densitometry (any time frame), DEXA, pQCT, torsional stiffness or maximum torque. Despite the lack of statistically relevant data, trends were observed with larger callus size and density in the dynamized tibiae. The dynamized tibiae appeared to fracture more consistently outside of the area of the healing callus as compared to the control tibiae. Histological evaluation showed greater remodelling in four of five control limbs when compared to the dynamized limb. Dynamization at 31 days post-operatively may delay bone remodelling, despite a trend towards a larger callus size. The results of this study failed to show a definitive role for early full axial dynamization.

Computational simulation of axial dynamization on long bone fractures

Background. Axial dynamization has been shown in previous studies to promote callus formation, improve bone healing at fracture sites, and enhance bone remodeling. However, the possibility of non-axial movements or uniform fracture site compression during dynamization, and the appropriate relaxation of fixator joints to achieve such function, have not been investigated. Methods. This study used previously developed computational models based on two commercially available unilateral external fixators (Dynafix and Orthofix) to analyze the fixator joint adjustments used and the fracture site movements generated during dynamization. Findings. When none of the fixator’s sliding joints were parallel to the long bone axis, significant non-axial movements occurred during dynamization. The dual sliding joint design of the Dynafix fixator was beneficial in reducing these non-axial movements. When all of the fixator joints were allowed to adjust simultaneously during dynamization, exact axial movement or uniform compression at a complicated fracture site was achievable. Interpretation. This study revealed that significant non-axial movements may occur during dynamization, and that such a deficiency can be corrected by relaxing certain fixator joints in addition to the sliding mechanism. The same modeling technique can also be applied in bone lengthening application to assure desirable limb alignment during the distraction process. These analysis results can aid the performance assessment of an external fixator and facilitate appropriate application of such a device to achieve either active or controlled axial movement.

Technique and considerations when using external fixation as a standard treatment of femoral fractures in children

Femoral fractures in children can be treated effectively and with a low complication rate by using external fixation. However, as with most treatment modalities there is a learning curve to be considered. The aim of this paper is to report “tricks” and different considerations that we have learned to be of value based on experience gained during a prospective and consecutive study of 98 femoral fractures in children aged 3–15 years. Our experience is based on the use of a unilateral fixator with the option to apply axial dynamisation. Traction prior to operation is not needed if the child is operated on within 24 h. During surgery a traction table will prevent significant malrotation and facilitate reduction prior to insertion of the pins. Four 4 or 5 mm pins are sufficient for adequate stability in children. Transverse skin incisions are preferable for pin insertion as the scars become smaller and the soft tissue irritation during activity is less when compared with longitudinal incisions. Unrestricted weight-bearing can be allowed. A nihilistic approach to pin site care with daily showers is as effective as more aggressive treatment with local antiseptics. Pin infections can occur but are mild and can be treated locally or with a short period of antibiotics taken orally. Pin-loosening and deep infections are very uncommon. By using external fixation, malunion, overgrowth or delayed union can almost be avoided completely. Re-fractures are rare and occur only after significant trauma. Treatment time is relatively short. No physiotherapy or further protection of the leg is necessary during or after healing.

Experimental study on the effect of mechanical stimulation on the early stage of fracture healing.

In an attempt to ascertain the effects of mechanical stimulation on callus in the early stage of bone fracture healing, a tibial fracture was induced in rats and mechanical stimulation applied to the fractures. The callus was then measured quantitatively, while the fractures were analyzed both radiographically and histologically. Following the induction of a closed transverse fracture in the tibia, external anchors were applied and the rats raised by suspending the fractured leg. The rats were divided into two main groups: a Stimulation Group (S Group) and a Control Group (C Group) without the application of any mechanical stimulation. The S Group was further divided into the following three subgroups: an axial compression group (Sc Group) receiving stimulation in the positive direction; an axial distraction group (Sd Group) receiving stimulation in the negative direction; and an axial dynamization group (Sdy Group) receiving stimulation in both directions alternately. For mechanical stimulation, 1.4-N sine waves were applied continuously for 30 minutes a day, three times a week, starting 2 days after fracture-inducing surgery. At 3, 7, and 14 days after surgery, transverse sections of each fractured bone sample were prepared. At 14 days after surgery, each transverse section was divided into two peripheral and central regions to permit calculation of the area ratio of callus. Radiographically, no marked differences were observed among the groups; histologically, differences were seen 7 days after surgery, suggesting that mechanical stimulation facilitated bone healing soon after surgery. At 14 days after surgery, the amount of callus for the C Group was less than that for all three stimulation groups. In the C Group, the amount of callus in the peripheral region was greater than in the central region, and in the Sc Group, the results were the same: callus in the peripheral region was greater than in the central region. In the Sd Group, callus was greater in the central region than in peripheral regions. In the Sdy Group, favorable callus was observed in both the central and peripheral regions. These findings suggest that axial compression facilitates callus primarily in the peripheral region, while axial distraction facilitates callus primarily in the central region. When axial compression and distraction were alternated (dynamization), callus was significantly facilitated in both the central and peripheral regions. Of the three axial stimulation techniques, dynamization was the most effective in facilitating callus in the early stage of bone fracture healing.

Comparison of dynamic versus static external fixation for pediatric femur fractures.

External fixation of pediatric femoral shaft fractures has the advantages of minimal dissection and early weight bearing. However, it is associated with slow healing and potential for refracture. Some surgeons have proposed that axial dynamization may improve the speed and strength of callus formation. to test this hypothesis, we performed a randomized controlled trial using 53 femur fractures in 52 patients between 1995 and 1999. Patients were randomized to receive dynamic or static fixation. Average time until early callus formation was 23.2 days for dynamic fixation and 24.9 days for static fixation (P = 0.627). Average time until complete radiographic healing was 70.1 days for dynamic fixation and 63.1 days for static fixation (P = 0.370). Similarly, the differences in time to fixator removal and to full weight bearing did not reach statistical significance. The conclusion was that axial dynamization of external fixation for pediatric femur fractures has no significant effect on time to healing or frequency of complications.

The Effects of Dynamization and Destabilization of the External Fixator on Fracture Healing: A Comparative Biomechanical Study in Dogs

This study compared the effects of axial dynamization and staged destabilization on fracture healing. Bilateral midshafts of canine tibiae were osteotomized and fixed with an external fixator. The hind limbs were divided into two groups: the destabilized group in which the fixator's stiffness was progressively reduced over time and the axially dynamized group in which the fixator was axially dynamized. The healed tibiae were tested for 3-point bending in the anteroposterior plane. The biomechanical tests performed 2 months postoperatively revealed that the side with the destabilized fixator was more rigid than the side with the axially dynamized fixator, but the differences were insignificant (P=.20). This study showed staged destabilization of the fixator's stiffness was as effective on the enhancement of fracture healing as axial dynamization.

Effect of early axial dynamization on tibial bone healing: a study in dogs.

Early axial dynamization and its effect on experimental tibial bone healing was compared with healing under rigid fixation in a time-sequenced manner using dogs. An external fixator that could be rigidly locked or set to allow free axial movement while preventing bending and shear was used. Both tibias were osteotomized and externally fixed, leaving a gap between bone ends of 2 mm. At 1 week, one side was dynamized, whereas the other side was kept rigidly locked as a control. Dogs were euthanized at 1 day and 1, 3, 5, 8, and 11 weeks after dynamization. The outcome measures were static and dynamic load-bearing, periosteal callus development, new bone formation, callus tissue composition, and mechanical strength. Load bearing was higher on the dynamized limbs during standing for the first 5 weeks and during gait for the first 3 weeks after dynamization compared with the controls. Maximum periosteal callus size was reached faster and was distributed more symmetrically on the dynamized side. The periosteal callus area decreased at 12 weeks on the dynamized sides, but there was no significant change in the area on the control sides. Endosteal new bone formation and bone density decreased between 9 and 12 weeks only on the dynamized sides. The dynamized side showed a significantly higher torsional stiffness at 6 weeks than did the controls. There were no significant differences between dynamized and control tibias at other times. Maximum torque also tended to be higher on the dynamized sides at the same time. Early axial dynamization appeared to accelerate callus formation and remodeling and to provide higher mechanical stiffness during early stages of bone healing.

Do axial dynamic fixators really produce axial dynamization?

The use of dynamic external fixators for the treatment of long bone fracture is widespread and well accepted. It is claimed that dynamization, i.e. small micromovements of compression/distraction at the fracture site, can be produced by allowing sliding of an inner rod within an outer housing. However, as the forces on the fixator are not direct but transferred from the bone via bone pins, there is a bending moment on the fixator. This produces “self-locking” and effectively prevents axial movements. We have studied the effect of this moment on the binding properties of the Orthofix system. The amount of movement at a simulated fracture site allowed before this locking occurs was measured and its implications discussed. It would appear that true axial dynamization does not take place using this system.