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Comparison of unreamed nailing and external fixation of tibial diastases––mechanical conditions during healing and biological outcome
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Journal of Ort hopa ed ic Research
Journal of Orthopaedic Research 22 (2004) 1072-1078
www.elsevier.com/locate/orthres
Comparison of unreamed nailing and external fixation of tibial diastases-mechanical conditions during healing and biological outcome P. Klein, M. Opitz, H. Schell, W.R. Taylor, M.O. Heller, J.-P. Kassi, F. Kandziora, G.N. Duda * Center ,for Musculoskeletal Surgery, Charitt.-University Medicine Berlin, Free and Humboldt- University of’ Berlin, Campus Virchow-Clinic, Augustenburger Platz 1, 0-13353 Berlin, Germany
Received 7 April 2003; accepted 10 February 2004
Abstract
Locked intramedullary nailing and external fixation are alternatives for the stabilization of tibial shaft fractures. The goal of this study was to determine to what extent the mechanical conditions at the fracture site influence the healing process after unreamed tibial nailing compared to external fixation. A standardized tibial diastasis was stabilized with either a locked unreamed tibial nail or a monolateral fixator in a sheep model. Interfragmentary movements and ground reaction parameters were monitored in vivo throughout the healing period. After sacrifice, the tibiae were examined mechanically and histologically. Bending angles and axial torsion at the fracture site were larger in the nail group within the first five weeks post-operatively. Unlike the fixator group, the operated limb in the nail group did not return to full weight bearing during the treatment period. Mechanical and histomorphometrical observations showed significantly inferior bone healing in the nail group compared to the fixator group. In this study, unreamed nailing of a tibial diastasis did not provide rotational stability of the osteosynthesis and resulted in a significant delay in bone healing. 0 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved. Keywordsst Fracture healing; Unreamed tibial nail; External fixator; Interfragmentary movements
Introduction Locked intramedullary nailing is the most frequently used osteosynthetic procedure for the stabilization of tibial shaft fractures [21]. Since tibial fractures are regularly a result of a high energy trauma, they are often multi-fragmentary and frequently associated with severe soft tissue injury [20,26]. Multi-fragment tibial fractures ( A 0 classification type B and C) are often (710/0)rotationally unstable [12]. Unstable tibial fractures are frequently treated by external fixators or unrearned tibial nails in which the unreamed tibial nail was originally designed as a temporary implant and as an alternative to external fixation [20,22]. Locking increases axial and rotational stability [15,16,19]. Elastic deformation of the
* Corresponding author. Tel.: +49-30-450-559079; fax: +49-30-450559969. E-mail address [email protected] (G.N. Duda).
nail, in combination with the play of the interlocking bolts, provides a mechanical stimulus at the fracture zone and has shown good clinical results even without precise fragment reduction [I 5,16,19]. It is widely accepted that next to biology and fracture related factors, mechanical conditions at the fracture site, represented by interfragmentary movements, influence both the type and rate of fracture healing [6,13,18]. The mechanical stiffness and the amount of play in the osteosynthesis determines the amount of movement allowable between the bone fragments and therefore has a major effect on the healing process and clinical outcome [7,17]. Rigid fixation and compressive immobilization does not allow relative movement between the fragments and leads to direct cortical union. Flexible fixation allows more interfragmentary movement and leads to callus formation [23]. Overly flexible osteosyntheses, however, extend the healing period or even delay bone healing [4,30,3 11. Axial interfragmentary movement within
0736-02666 - see front matter 0 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved doi: 10.1016/j.orthres.2004.02.006
P. Klrin et ul. I Journal of Orthopardie Research 22 (2004) 1072-1078
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certain limits, however, is capable of promoting callus formation [5,13]. Conclusions regarding shear seem to be unclear and the literature suggests that it should either be avoided or may enhance fracture healing by promoting “callus expansion” and osteogenesis [32]. The goal of this study was to determine the mechanical environment after unreamed tibial nailing compared to external fixation and to evaluate to what extent the mechanical conditions at the fracture site influence the healing process. A not precisely reduced multi-fragmentary fracture should be simulated by creating a gap situation in a sheep model. The mechanical environment should be described by interfragmentary movements at the fracture site throughout the entire healing period. In addition, the influence on the long term biological outcome should be demonstrated for the two different osteosyntheses by histological and biomechanical results.
Methods A niinals
Prior to inclusion, all sheep were screened radiographically for the width of their tibial marrow cavity. Therefore, the animals were fixed in a right lateral position and standardized mediolateral radiographs (70 kV, 2.5 mAs, film-focus distance 100 cm) of the right tibia were taken. Twelve female merino-mix sheep (average 2 years) were randomly divided into two groups of six sheep (mean weight 77 kg, range 59-90 kg). All animal experiments were carried out according to the policies and principles established by the Animal Welfare Act, the NIH Guide for Care and Use of Laboratory Animals and the national animal welfare guidelines. The study was approved and monitored by the local legal representative.
’
Pre-operative gait analyses
All animals were trained to walk over a gangway, which contained a pressure sensitive platform (emed SF-4, novel, Munich, Germany). Ground reaction parameters were recorded the day before operation for all legs using the first step method. A measurement cycle included a single forelimb and its corresponding hind limb. Seven successful trials per limb per animal were taken and averaged. Surgical procedure
All sheep underwent a standardized midshaft osteotomy of the right tibia which was stabilized with either an unreamed tibial nail (UTN group, Fig. I , left) or a monolateral external fixator (external fixator group, Fig. I , right). For the UTN group, a commercially available unreamed tibia nail (UTN, 0 9 mm, No. 479.250. Synthes, Bochum, Germany) was shortened to 210 mm in order to fit t o the length of a sheep tibia. The nail was inserted and pulled back halfway. The tibia was osteotomized diaphyseally with a n oscillating saw and the osteotomy gap was distracted to 3 mm using a guide system. The nail was again completely inserted and statically locked with four mediolaterally inserted locking bolts (@ 3.9 mm, No. 458.200-800, Synthes, Bochum, Germany). In the fixator group, a monolateral external fixator was mounted 5 mm, 3 anteromedially. The fixator consisted of 6 Schanz’ screws (0 inserted proximally, 3 inserted distally to the osteotomy) and 2 carbon
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Landesamt fur Arbeitsschutz. Gesundheitsschutz und technische Sicherheit. Berlin: G 0188/99.
Fig. 1. Anteroposterior view of an osteosynthesis of a right sheep tibia with an unreamed tibial nail (left) and external fixator (right). The nail was statically locked with four mediolaterally orientated locking bolts. Additional Schanz’ screws were inserted for the measurement of interfragmentary movements, to which light triangular frames were attached during measurement sessions, each holding three reflective markers. These screws were orientated in the anteroposterior direction of the tibia. 10 mm). Subsequent to the fixator mounting, the tibia fibre rods (0 was osteotomized diaphyseally. The distal clamping jaws were loosened and the osteotomy gap was distracted to 3 mm using a guide system. In all cases, the soft tissue was preserved to exclude influences of tissue damage on the process of bone healing and establish standardized experimental conditions. T o measure the relative movements of the bone fragments. two additional Schanz’ screws were implanted, one proximal and one distal. A guide system was used t o standardize their insertion at a distance of 35 mm from the osteotomy in all animals. Both screws were orientated in the anteroposterior direction of the tibia. In the UTN group these screws were inserted eccentrically to avoid any interference with the nail. Thus, these screws were implanted monocortically, on the secant of the medial tibial cortices (Fig. I). In the fixator group, the screws were implanted bi-cortically and not connected with the fixator frame. In both cases, these screws remained unloaded and did not interfere with the fixation device in either group. To achieve identical mechanical conditions for the measurement of interfragmentary movements in the two groups, the distance between the bony pin entry and the adapter for the triangles with the reflective markers was kept identical in all animals. The skin was sutured and the leg, together with the external fixator, was covered with a tube bandage. Mediolateral and anteroposterior radiographs were taken directly postoperatively.
Post-operative cure
All sheep received an analgesic for three days post-operatively (Finadyne’, Essex, Munich, Germany). Daily pin care was performed by cleaning the insertion sites of the Schanz’ screws with Ethacridinlactate (Rivanol@, 5%, Chinosol, Germany). The clinical course of healing was monitored by taking anteroposterior radiographs of the affected limb in an unloaded position at weekly intervals.
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hterjragmentury movement and ground reaction parameters
Histomorphometric analysis
The sheep were walked over the gangway weekly for a nine week period, beginning two to three days post-operatively. Limb loading was monitored with the pressure sensitive platform in the same manner as before surgery. A full measurement cycle began at the heel strike of the forelimb and ended with toe-off of its corresponding hindlimb. Depending on the cooperation of the sheep, a minimum of three and up to seven trials per limb and measurement session were taken and averaged. During measurement sessions, light triangular frames were attached to the additional Schanz' screws in both bone fragments, each holding three reflective markers. The positions of each marker was recorded using an infrared optical measurement system (PCReflex, Qualisys, Sweden; accuracy of measurements: f O . l mm, t0.1") at a frequency of 60 Hz. From the post-operative radiographs. the threedimensional offsets of the reflective markers to the center of the fracture gap were measured. Using these offsets, the measured relative movements of the reflective markers were translated into interfragmentary movements at the fracture site using matrix algebra. These movements were given as axial compression [mm], shear [mm] and axial torsion ["I. In addition, the bending angles around the anteroposterior and mediolateral axes were reported.
Directly after biomechanical testing, all callus regions were cut in the frontal plane. The anterior portion was embedded in methylmethacrylate (Technovitm 9100, Heraeus Kulzer, Germany) and 3 mm serial histological sections were cut. These sections were stained with Safranin Orangehon Kossa or Safranin Orange/Fast Green for differentiation of callus tissues. Computerized histomorphometric analysis was performed with an image analysis system (KS400, Zeiss, Germany). The region of interest contained the gap plus twice the width of the gap in the proximal and distal directions. The quality and quantity of the callus tissue was examined with respect to bone, cartilage and fibrous tissue formation [2]. Tissue differentiation was analyzed for various locations within the callus (endosteal, periosteal, lateral and medial). All histological measurements were performed by two experienced independent observers and all results were averaged.
Bionwchanic.ul testing of the nine week cullus All animals were sacrificed nine weeks after surgery, since earlier studies documented sufficient bone healing for the external fixator chosen in the present study [32]. The fixators were then removed and the intramedullary nails extracted. Both, the left and right tibiae were dissected, leaving muscles which were attached to the tibia in place. Thus, the in vitro configuration reflected best the osteosynthesis stiffness in vivo and callus tissue was best preserved for histological analysis. For biomechanical testing, the tibial muscles were covered with a mull bandage moisturized with NaCl 0.9%. The proximal and distal ends of each tibia were embedded in acrylate (Beracryl, W. Troller AG, Switzerland). Using a material testing machine (Zwick 1445, Ulm, Germany), torsional testing was performed until failure at the rate of I0"imin with an axial pre-load of 20 N. The torsional strength (maximum torque to failure [Ncm]) and torsional stiffness [N cmP] were determined. Biomechanical parameters were then reported as a percentage of the intact contralateral side.
Determination of ihe three-dimensional siiffness of the fixation devices in vitro
In a separate analysis, the stiffness of each type of osteosynthesis was determined. Six cadaveric unpaired tibiae per construct type were implanted with a UTN or an external fixator using the surgical procedures described in the main study. The tibial muscles were left in place and the tibiae were prepared for testing in the material testing machine. All bone-implant constructs were loaded in six load cases (axial compression, torsion, bending and shear in two planes) [14]. The axial stiffness, the shear and the bending stiffness in the anteroposterior and mediolateral directions, together with the torsional stiffness were calculated for each bone-implant construct. Finally, the stiffness in each direction for each type of implant osteosynthesis was averaged across the six tested specimens (Table 1). Statistical analysis Statistical analysis between the groups was performed using the Mann-Whitney U-test for unpaired data (SPSS 9.0, SPSS Inc., Chicago, IL). Statistical comparisons between the pre- and post-operative data of one limb were performed using the Wilcoxon test for connected data. A p-value of less than 0.05 was regarded as showing a significant difference.
Table 1 In vitro stiffness data (Panel A) and interfragmentary movements measured in vivo (Panel B) Unreamed tibial nail Mean Std-dev
Std-dev (%)
External fixator Mean Std-dev
Std-dev (%)
Mann-Whitney U-test @-value) ~
Panel A Load case Axial compression [N/mm] 1775.8 Axial torsion [N m/"] 1.4 m/l shear [N/mm] 139.2 alp bending [N/mm] 114.7 m/l bending [N d o ] 16.2 a/p bending [N do] 9.9
1314.6 0.9 58.5 72.8 14.3 2.3
74.0 66.4 42.0 63.5 88.2 23.4
1958.9 3.4 206.6 350.2 32.4 21.4
633.5 0.3 80.2 122.6 9.6 34.6
29.2 10.4 33.5 37.4 27.8 94.6
0.065 (n.s.) 0.004 0.009 0.026 0.041 0.002
Panel B Initial interfragmentary movements Axial compression [mm] 1.8 1.4 78.1 1 0.1 11.9 0.53 (n.s.) Axial torsion ["I 8.1 4.8 54. I 2.7 0.6 22.6 0.035 Shear [mm] 2.5 1.6 62.1 1.7 0.4 24.4 0.37 (n.s.) m/l bending ["I 2.7 2.1 70.8 0.7 0.3 36.7 0.001 a/p bending ["I 2.4 1.2 49.2 1.1 0.3 29.9 0.005 In vitro stiffness data for the UTN and the EF group. Mean and standard deviation are given for six different load cases. All tibial muscles were left in place (Panel A). In vivo measured interfragmentary movements two days post-operation for the UTN and the EF group. Axial torsion and bending angles were significantly larger in the unreamed tibial nail group compared to the external fixator group whilst axial compression and shear movement showed a strong but not significant trend to be larger in the UTN group. The standard deviation was generally increased in the UTN group compared to the EF group (n.s. = n o significant difference) (Panel B).
P. Klein et al. I Journal of Orthopaedir Research 22 (2004) 1072-1078
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Results The marrow cavity widths ranged from 9.6 to 10.7 mm at their narrowest point. This meant a conformity between unreamed tibia1 nail and bone of 0.6 up to 1.7 mm. No correlations between the conformity and the initial interfragmentary movements were, however, observed. The in vivo analyses showed significantly larger interfragmentary axial torsion in the UTN group for the first five weeks post-operatively (Fig. 2, p ,< 0.035). In addition, anteroposterior and mediolateral bending angles showed significant differences between the groups for the first five weeks (p < 0.005 and p < 0.004) (Fig. 3). No statistical differences could be observed for the amounts of axial compression and shear movement (Table 1, Panel B). Interfragmentary movements decreased throughout the healing period. The axial torsion of the two groups converged at five weeks post-operatively (Fig. 2). In both groups, the sheep unloaded the operated hindlimb (Fig. 4) and a simultaneous overloading of the contralateral hindlimb was observed: Although both the contact force and the contact area of the operated limb decreased, the contact time of both hind limbs increased, demonstrating a reduced walking speed. The unloading of the affected limb was more pronounced in the UTN group (p < 0.009). Whilst the animals in the external fixator group returned to normal weight bearing within 60 days, no return to normal weight bearing could be observed in the UTN group within the nine week period. In fact, the unloading of the operated limb consistently increased until 35 days post-operatively in the UTN group (Fig. 4). Biomechanical testing of the callus postmortem showed significantly less torsional strength (p = 0.005) in combination with less torsional stiffness (p = 0.035) for
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-External Fixator t u n r e a m e d Tibial Nail
37 u 0,
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20 30 40 days post operation
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Fig, 3. Mediolateral (top) and anteroposterior bending (bottom) at the center of the fracture site throughout the healing period (mll bending: p* = 0.001, p** = 0.004, p*** = 0.001, p"'* = 0.004; alp bending: p * = 0.005,p"=0.004,p**'=0.001,p***'=0.041).
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6External Fixator
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Fig, 4. Maximum ground reaction force of the right hindlimb throughout the healing period, shown as a percentage of the preoperative values @" = 0.005, p * 2= 0.008, p*3= 0.009, p'4 = 0.004, p*' = 0.001, p*6= 0.004, p*' = 0.003). 0
10
20 30 40 days post operation
50
60
. ^. . . Fig. 2. Axial torsion throughout the healing period of the proximal in .
. ..
.
respect to the distal fragment is significantly smaller in the external fixator group @* = 0.035), p'* = 0.004, p***= 0.005, p*"* = 0.002).
the UTN group compared to the external fixator group: In the UTN group the torsional strength amounted to 50.3 k 16.5% of the intact contralateral side versus 78.5 k 11% in the E F group whilst the average torsional
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P. Klein et a1 I Journal of Orthopaedic Research 22 (2004) 1072-1078
stiffness amounted to 69.3+_27%in the UTN group versus 91.4k7.1 in the EF group. Histologically, the callus tissue appeared to be nonuniform in the UTN group, with osseous regions surrounded predominantly by fibrous tissue (Fig. 5 , top). Complete periosteal bone bridging of the gap could only be observed laterally, whilst on the medial side, bone bridging was incomplete. Osseous filling of the intercortical gap did not occur in the UTN group. In the external fixator group the periosteal callus was delimited, bulbous and pronounced on the medial side. In contrast to the UTN group where the nail filled mostly the medullary space, the intercortical gap was completely bridged with mineralized bone tissue in the external fixator group (Fig. 5 , bottom). The total width of the callus (p==0.002) as well as the mineralized bone width (p == 0.002) were significantly lower in the UTN group. In the same group, the thickness of the cortices was reduced, and significantly less cortical bone (p = 0.001) and cortical mineralized bone (p = 0.002) could be seen. In addition, the bone perimeter was significantly reduced (p == 0.002) when compared with the external fixator group.
Fig. 5. Comparison of histological results: Safranin-Orangehon Kossa staining of an osteotomy site nine weeks after unreamed nailing (top) and external fixation (bottom). The medial cortical bone is on the lefthand side of the images.
The in vitro biomechanical testing showed that the unreamed tibial nail provided significantly less torsional stiffness than the external fixator (Table 1, Panel A, p = 0.004). No statistical differences could be observed for the amounts of axial compression, whilst the anteroposterior and mediolateral shear stiffness’ (p = 0.026 and p = 0.009 respectively), and the anteroposterior and mediolateral bending stiffness’ differed significantly (p = 0.002 and p = 0.041 respectively) between the bone-implant constructs in vitro. Interindividual variations were generally larger in the UTN group in these in vitro tests.
Discussion The goal of this study was to determine the initial mechanical environment in a gap situation, simulating a not reduced fracture with unreamed tibial nailing compared to external fixation and to demonstrate the influence of the mechanical environment on the long term biological outcome in a sheep model in vivo. Comparison of two types of osteosyntheses in sheep has shown a greater unloading of the operated limb and a less developed callus formation in the animals treated with an unreamed tibial nail. The animals treated with the external fixator showed significantly less interfragmentary movement in the early stages of healing and returned to full weight bearing within 60 days postoperatively, suggesting near complete healing of the fracture. This observation was supported by the postmortem mechanical stiffness of the callus which returned to within 20% of the contralateral intact tibia. Loading of the operated limb in the UTN group, however, progressively decreased until week five, after which loading began to increase. At nine weeks post-operatively, the UTN group sheep were still not fully loading the operated limb, and the biomechanical as well as histomorphometrical differences in callus quality were still significant for healing. It is known that functional weight bearing is able to accelerate the rate of fracture healing and thus to significantly improve the strength of the healing bone [25]. However, early, full weight bearing with flexible fixation has shown delayed fracture healing [l]. In addition, it has been reported that the maximum displacement at the fracture site occurred when the affected limb was unloaded and that partial weight bearing, as described in clinical situations, does not necessarily lead to an unloading of the affected limb [10,24]. From this, it seems that the initial unloading of the limb may have had a long term influence on callus healing, possibly causing a slower healing rate compared to the fixator group. Since the initial stability of the osteosynthesis is a crucial factor for the success of treatment, the larger movements in the UTN group may have caused a
P. Klein et al. I Journal qf’Orthopuedic Research 22 (2004) 1072-1078
delayed callus formation compared to the fixator group [6,13,18]. The post-operative axial torsion, as well as the mediolateral and anteroposterior bending angles, were significantly larger in vivo within the first five weeks, most likely as a direct consequence of the different stiffness’ of the bone-implant constructs. Compared to other animal experimental analyses, the interfragmentary movements found for the fixator group in this study were in the described range which has been found to be optimal for fracture healing throughout the entire healing period. In contrast, in the UTN group the interfragmentary movements had decreased to within limits that are not deemed acceptable until nine weeks post-operation [5,13]. Histomorphometrical analysis showed obvious differences in morphology of the callus between the two groups. The periosteal callus showed considerable differences, also representing delayed callus formation for the unreamed nail group. These histomorphometrical findings were closely supported by the results of reduced biomechanical stiffness of the fracture callus in the UTN group. In addition, histology of the UTN group displayed signs of cortical thinning [3,1 I]. The bone loss of the diaphyseal cortex is thought to be a result of the rigid nail in combination with the large interfragmentary movements observed in this group, since rigid implants were described to lead to a necrosis of the cortical bone due to the stress to which it was exposed [3,8,29]. For the unreamed nail, the in vitro determined stiffness of the bone-implant constructs using sheep tibiae was comparable with those reported for human tibiae [27,28]. The poor healing observed for the UTN group in this study has not been reported for clinical situations so far. This might be attributed to differences in anatomy (e.g. absence of the fibula in sheep) and musculo-skeletal loading conditions that further determined the biological healing process [9]. Due to the experimental nature of both the defect situation as well as the osteosynthesis, the findings of the present study may not be able to be directly transferred into clinical situations. Nevertheless, some general conclusions should be drawn. In a clinical setting, it is well known that fracture gaps of more than 3 mm should be avoided in osteosyntheses with unreamed tibial nailing due to the risk of delayed healing (by 12 times) or non-unions (fourfold) [ 121. Therefore, this sheep model confirmed the recommendation of a precise reduction of the fracture fragments as a pre-requisite for good fracture healing. The data of the present study illustrate that otherwise the mechanical conditions in locked unreamed nailing may lead to delayed fracture healing. The unreamed tibial nailing could be further optimized by increasing the in vivo stability, particularly the axial torsional stiffness of the osteosynthesis. If it were possible to mimic the rotational stiffness of the presented external fixation in the bone-nail construct, then it may
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be possible to combine the advantages of the locked unreamed nailing with the rapid healing rates of the more stable osteosynthetic treatment options. This could help to avoid the severe mechanical conditions which seem to be detrimental to the process of bone healing.
Acknowledgements This study was partially supported by a grant of the A 0 Foundation Davos and the German Research Foundation (DFG K F G 102/1). The authors would like to thank Prof. S. Perren, Davos, Switzerland for editing this manuscript and Prof. Dr. M. Raschke, Muenster, Germany for clinical support.
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[I 51 Kempf 1, Grosse A, Beck G. Closed locked intramedullary nailing. Its application to comminuted fractures of the femur. J Bone Joint Surg Am 1985;67:709-20. [ 161 Kempf I, Grosse A, Taglang G, Bernhard L, Moui Y. Interlocking central medullary nailing of recent femoral and tibial fractures. Statistical study apropos of 835 cases. Chirurgie 1991;117:478-87. [I71 Kenwright J, Goodship AE. Controlled mechanical stimulation in the treatment of tibial fractures. Clin Orthop 1989:3647. [18] Kenwright J, Richardson JB, Cunningham JL, et al. Axial movement and tibial fractures. A controlled randomised trial of treatment. J Bone Joint Surg [Br] 1991;73:654-9. [I91 Klemm K, Schellmann WD, Vittali HP. Intramedullary nail bolted to the femur and tibia. Bull SOCInt Chir 1975;34:93-6. [20] Krettek C, Haas N, Schandelmaier P, Frigg R, Tscherne H. Unreamed tibial nail in tibial shaft fractures with severe soft tissue damage. Initial clinical experiences. Unfallchirurg 1991;94:579-87. [21] Krettek C, Konemann B, Miclau T, et al. A new technique for the distal locking of solid AO unreamed tibial nails. J Orthop Trauma 1997;1 1 :446-5 1. [22] Krettek C, Schandelmaier P, Rudolf J, Tscherne H. Current status of surgical technique for unreamed nailing of tibial shaft fractures with the UTN (unreamed tibia nail). Unfallchirurg 1994;97:57599. [23] Perren SM. Biomechanical basis of fracture treatment. Orthopade 1992;21:3- 10.
[24] Sarmiento A, McKellop HA, Llinas A, et al. Effect of loading and fracture motions on diaphyseal tibial fractures. J Orthop Res 1996;14:804. [25] Sarmiento A, Schaeffer JF, Beckerman L, Latta LL, Enis JE. Fracture healing in rat femora as affected by functional weightbearing. J Bone Joint Surg [Am] 1977;59:369-75. [26] Schandelmaier P, Krettek C, Rudolf J, Tscherne H. Outcome of tibial shaft fractures with severe soft tissue injury treated by unreamed nailing versus external fixation. J Trauma 1995;39:70711. [27] Schandelmaier P, Krettek C, Tscherne H. Biomechanical studies of 9 tibial interlocking nails in a bone-implant unit. Unfallchirurg 1994;97:60&8. [28] Schandelmaier P, Krettek C, Tscherne H. Biomechanical study of nine different tibia locking nails. J Orthop Trauma 1996;10:3744. [29] Uhthoff HK, Dubuc FL. Bone structure changes in the dog under rigid internal fixation. Clin Orthop 1971;81:165-70. [30] Williams EA, Rand JA, An KN, Chao EY, Kelly PJ. The early healing of tibial osteotomies stabilized by one-plane or two-plane external fixation. J Bone Joint Surg Am 1987;69:35545. [31] Wu JJ, Shyr HS, Chao EY, Kelly PJ. Comparison of osteotomy healing under external fixation devices with different stiffness characteristics. J Bone Joint Surg Am 1984;66:1258-64. [32] Yamagishi M, Yoshimura Y. The biomechanics of fracture healing. J Bone Joint Surg 1955;37-A:1035-68.
Journal of Ort hopa ed ic Research
Journal of Orthopaedic Research 22 (2004) 1072-1078
www.elsevier.com/locate/orthres
Comparison of unreamed nailing and external fixation of tibial diastases-mechanical conditions during healing and biological outcome P. Klein, M. Opitz, H. Schell, W.R. Taylor, M.O. Heller, J.-P. Kassi, F. Kandziora, G.N. Duda * Center ,for Musculoskeletal Surgery, Charitt.-University Medicine Berlin, Free and Humboldt- University of’ Berlin, Campus Virchow-Clinic, Augustenburger Platz 1, 0-13353 Berlin, Germany
Received 7 April 2003; accepted 10 February 2004
Abstract
Locked intramedullary nailing and external fixation are alternatives for the stabilization of tibial shaft fractures. The goal of this study was to determine to what extent the mechanical conditions at the fracture site influence the healing process after unreamed tibial nailing compared to external fixation. A standardized tibial diastasis was stabilized with either a locked unreamed tibial nail or a monolateral fixator in a sheep model. Interfragmentary movements and ground reaction parameters were monitored in vivo throughout the healing period. After sacrifice, the tibiae were examined mechanically and histologically. Bending angles and axial torsion at the fracture site were larger in the nail group within the first five weeks post-operatively. Unlike the fixator group, the operated limb in the nail group did not return to full weight bearing during the treatment period. Mechanical and histomorphometrical observations showed significantly inferior bone healing in the nail group compared to the fixator group. In this study, unreamed nailing of a tibial diastasis did not provide rotational stability of the osteosynthesis and resulted in a significant delay in bone healing. 0 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved. Keywordsst Fracture healing; Unreamed tibial nail; External fixator; Interfragmentary movements
Introduction Locked intramedullary nailing is the most frequently used osteosynthetic procedure for the stabilization of tibial shaft fractures [21]. Since tibial fractures are regularly a result of a high energy trauma, they are often multi-fragmentary and frequently associated with severe soft tissue injury [20,26]. Multi-fragment tibial fractures ( A 0 classification type B and C) are often (710/0)rotationally unstable [12]. Unstable tibial fractures are frequently treated by external fixators or unrearned tibial nails in which the unreamed tibial nail was originally designed as a temporary implant and as an alternative to external fixation [20,22]. Locking increases axial and rotational stability [15,16,19]. Elastic deformation of the
* Corresponding author. Tel.: +49-30-450-559079; fax: +49-30-450559969. E-mail address [email protected] (G.N. Duda).
nail, in combination with the play of the interlocking bolts, provides a mechanical stimulus at the fracture zone and has shown good clinical results even without precise fragment reduction [I 5,16,19]. It is widely accepted that next to biology and fracture related factors, mechanical conditions at the fracture site, represented by interfragmentary movements, influence both the type and rate of fracture healing [6,13,18]. The mechanical stiffness and the amount of play in the osteosynthesis determines the amount of movement allowable between the bone fragments and therefore has a major effect on the healing process and clinical outcome [7,17]. Rigid fixation and compressive immobilization does not allow relative movement between the fragments and leads to direct cortical union. Flexible fixation allows more interfragmentary movement and leads to callus formation [23]. Overly flexible osteosyntheses, however, extend the healing period or even delay bone healing [4,30,3 11. Axial interfragmentary movement within
0736-02666 - see front matter 0 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved doi: 10.1016/j.orthres.2004.02.006
P. Klrin et ul. I Journal of Orthopardie Research 22 (2004) 1072-1078
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certain limits, however, is capable of promoting callus formation [5,13]. Conclusions regarding shear seem to be unclear and the literature suggests that it should either be avoided or may enhance fracture healing by promoting “callus expansion” and osteogenesis [32]. The goal of this study was to determine the mechanical environment after unreamed tibial nailing compared to external fixation and to evaluate to what extent the mechanical conditions at the fracture site influence the healing process. A not precisely reduced multi-fragmentary fracture should be simulated by creating a gap situation in a sheep model. The mechanical environment should be described by interfragmentary movements at the fracture site throughout the entire healing period. In addition, the influence on the long term biological outcome should be demonstrated for the two different osteosyntheses by histological and biomechanical results.
Methods A niinals
Prior to inclusion, all sheep were screened radiographically for the width of their tibial marrow cavity. Therefore, the animals were fixed in a right lateral position and standardized mediolateral radiographs (70 kV, 2.5 mAs, film-focus distance 100 cm) of the right tibia were taken. Twelve female merino-mix sheep (average 2 years) were randomly divided into two groups of six sheep (mean weight 77 kg, range 59-90 kg). All animal experiments were carried out according to the policies and principles established by the Animal Welfare Act, the NIH Guide for Care and Use of Laboratory Animals and the national animal welfare guidelines. The study was approved and monitored by the local legal representative.
’
Pre-operative gait analyses
All animals were trained to walk over a gangway, which contained a pressure sensitive platform (emed SF-4, novel, Munich, Germany). Ground reaction parameters were recorded the day before operation for all legs using the first step method. A measurement cycle included a single forelimb and its corresponding hind limb. Seven successful trials per limb per animal were taken and averaged. Surgical procedure
All sheep underwent a standardized midshaft osteotomy of the right tibia which was stabilized with either an unreamed tibial nail (UTN group, Fig. I , left) or a monolateral external fixator (external fixator group, Fig. I , right). For the UTN group, a commercially available unreamed tibia nail (UTN, 0 9 mm, No. 479.250. Synthes, Bochum, Germany) was shortened to 210 mm in order to fit t o the length of a sheep tibia. The nail was inserted and pulled back halfway. The tibia was osteotomized diaphyseally with a n oscillating saw and the osteotomy gap was distracted to 3 mm using a guide system. The nail was again completely inserted and statically locked with four mediolaterally inserted locking bolts (@ 3.9 mm, No. 458.200-800, Synthes, Bochum, Germany). In the fixator group, a monolateral external fixator was mounted 5 mm, 3 anteromedially. The fixator consisted of 6 Schanz’ screws (0 inserted proximally, 3 inserted distally to the osteotomy) and 2 carbon
’
Landesamt fur Arbeitsschutz. Gesundheitsschutz und technische Sicherheit. Berlin: G 0188/99.
Fig. 1. Anteroposterior view of an osteosynthesis of a right sheep tibia with an unreamed tibial nail (left) and external fixator (right). The nail was statically locked with four mediolaterally orientated locking bolts. Additional Schanz’ screws were inserted for the measurement of interfragmentary movements, to which light triangular frames were attached during measurement sessions, each holding three reflective markers. These screws were orientated in the anteroposterior direction of the tibia. 10 mm). Subsequent to the fixator mounting, the tibia fibre rods (0 was osteotomized diaphyseally. The distal clamping jaws were loosened and the osteotomy gap was distracted to 3 mm using a guide system. In all cases, the soft tissue was preserved to exclude influences of tissue damage on the process of bone healing and establish standardized experimental conditions. T o measure the relative movements of the bone fragments. two additional Schanz’ screws were implanted, one proximal and one distal. A guide system was used t o standardize their insertion at a distance of 35 mm from the osteotomy in all animals. Both screws were orientated in the anteroposterior direction of the tibia. In the UTN group these screws were inserted eccentrically to avoid any interference with the nail. Thus, these screws were implanted monocortically, on the secant of the medial tibial cortices (Fig. I). In the fixator group, the screws were implanted bi-cortically and not connected with the fixator frame. In both cases, these screws remained unloaded and did not interfere with the fixation device in either group. To achieve identical mechanical conditions for the measurement of interfragmentary movements in the two groups, the distance between the bony pin entry and the adapter for the triangles with the reflective markers was kept identical in all animals. The skin was sutured and the leg, together with the external fixator, was covered with a tube bandage. Mediolateral and anteroposterior radiographs were taken directly postoperatively.
Post-operative cure
All sheep received an analgesic for three days post-operatively (Finadyne’, Essex, Munich, Germany). Daily pin care was performed by cleaning the insertion sites of the Schanz’ screws with Ethacridinlactate (Rivanol@, 5%, Chinosol, Germany). The clinical course of healing was monitored by taking anteroposterior radiographs of the affected limb in an unloaded position at weekly intervals.
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hterjragmentury movement and ground reaction parameters
Histomorphometric analysis
The sheep were walked over the gangway weekly for a nine week period, beginning two to three days post-operatively. Limb loading was monitored with the pressure sensitive platform in the same manner as before surgery. A full measurement cycle began at the heel strike of the forelimb and ended with toe-off of its corresponding hindlimb. Depending on the cooperation of the sheep, a minimum of three and up to seven trials per limb and measurement session were taken and averaged. During measurement sessions, light triangular frames were attached to the additional Schanz' screws in both bone fragments, each holding three reflective markers. The positions of each marker was recorded using an infrared optical measurement system (PCReflex, Qualisys, Sweden; accuracy of measurements: f O . l mm, t0.1") at a frequency of 60 Hz. From the post-operative radiographs. the threedimensional offsets of the reflective markers to the center of the fracture gap were measured. Using these offsets, the measured relative movements of the reflective markers were translated into interfragmentary movements at the fracture site using matrix algebra. These movements were given as axial compression [mm], shear [mm] and axial torsion ["I. In addition, the bending angles around the anteroposterior and mediolateral axes were reported.
Directly after biomechanical testing, all callus regions were cut in the frontal plane. The anterior portion was embedded in methylmethacrylate (Technovitm 9100, Heraeus Kulzer, Germany) and 3 mm serial histological sections were cut. These sections were stained with Safranin Orangehon Kossa or Safranin Orange/Fast Green for differentiation of callus tissues. Computerized histomorphometric analysis was performed with an image analysis system (KS400, Zeiss, Germany). The region of interest contained the gap plus twice the width of the gap in the proximal and distal directions. The quality and quantity of the callus tissue was examined with respect to bone, cartilage and fibrous tissue formation [2]. Tissue differentiation was analyzed for various locations within the callus (endosteal, periosteal, lateral and medial). All histological measurements were performed by two experienced independent observers and all results were averaged.
Bionwchanic.ul testing of the nine week cullus All animals were sacrificed nine weeks after surgery, since earlier studies documented sufficient bone healing for the external fixator chosen in the present study [32]. The fixators were then removed and the intramedullary nails extracted. Both, the left and right tibiae were dissected, leaving muscles which were attached to the tibia in place. Thus, the in vitro configuration reflected best the osteosynthesis stiffness in vivo and callus tissue was best preserved for histological analysis. For biomechanical testing, the tibial muscles were covered with a mull bandage moisturized with NaCl 0.9%. The proximal and distal ends of each tibia were embedded in acrylate (Beracryl, W. Troller AG, Switzerland). Using a material testing machine (Zwick 1445, Ulm, Germany), torsional testing was performed until failure at the rate of I0"imin with an axial pre-load of 20 N. The torsional strength (maximum torque to failure [Ncm]) and torsional stiffness [N cmP] were determined. Biomechanical parameters were then reported as a percentage of the intact contralateral side.
Determination of ihe three-dimensional siiffness of the fixation devices in vitro
In a separate analysis, the stiffness of each type of osteosynthesis was determined. Six cadaveric unpaired tibiae per construct type were implanted with a UTN or an external fixator using the surgical procedures described in the main study. The tibial muscles were left in place and the tibiae were prepared for testing in the material testing machine. All bone-implant constructs were loaded in six load cases (axial compression, torsion, bending and shear in two planes) [14]. The axial stiffness, the shear and the bending stiffness in the anteroposterior and mediolateral directions, together with the torsional stiffness were calculated for each bone-implant construct. Finally, the stiffness in each direction for each type of implant osteosynthesis was averaged across the six tested specimens (Table 1). Statistical analysis Statistical analysis between the groups was performed using the Mann-Whitney U-test for unpaired data (SPSS 9.0, SPSS Inc., Chicago, IL). Statistical comparisons between the pre- and post-operative data of one limb were performed using the Wilcoxon test for connected data. A p-value of less than 0.05 was regarded as showing a significant difference.
Table 1 In vitro stiffness data (Panel A) and interfragmentary movements measured in vivo (Panel B) Unreamed tibial nail Mean Std-dev
Std-dev (%)
External fixator Mean Std-dev
Std-dev (%)
Mann-Whitney U-test @-value) ~
Panel A Load case Axial compression [N/mm] 1775.8 Axial torsion [N m/"] 1.4 m/l shear [N/mm] 139.2 alp bending [N/mm] 114.7 m/l bending [N d o ] 16.2 a/p bending [N do] 9.9
1314.6 0.9 58.5 72.8 14.3 2.3
74.0 66.4 42.0 63.5 88.2 23.4
1958.9 3.4 206.6 350.2 32.4 21.4
633.5 0.3 80.2 122.6 9.6 34.6
29.2 10.4 33.5 37.4 27.8 94.6
0.065 (n.s.) 0.004 0.009 0.026 0.041 0.002
Panel B Initial interfragmentary movements Axial compression [mm] 1.8 1.4 78.1 1 0.1 11.9 0.53 (n.s.) Axial torsion ["I 8.1 4.8 54. I 2.7 0.6 22.6 0.035 Shear [mm] 2.5 1.6 62.1 1.7 0.4 24.4 0.37 (n.s.) m/l bending ["I 2.7 2.1 70.8 0.7 0.3 36.7 0.001 a/p bending ["I 2.4 1.2 49.2 1.1 0.3 29.9 0.005 In vitro stiffness data for the UTN and the EF group. Mean and standard deviation are given for six different load cases. All tibial muscles were left in place (Panel A). In vivo measured interfragmentary movements two days post-operation for the UTN and the EF group. Axial torsion and bending angles were significantly larger in the unreamed tibial nail group compared to the external fixator group whilst axial compression and shear movement showed a strong but not significant trend to be larger in the UTN group. The standard deviation was generally increased in the UTN group compared to the EF group (n.s. = n o significant difference) (Panel B).
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Results The marrow cavity widths ranged from 9.6 to 10.7 mm at their narrowest point. This meant a conformity between unreamed tibia1 nail and bone of 0.6 up to 1.7 mm. No correlations between the conformity and the initial interfragmentary movements were, however, observed. The in vivo analyses showed significantly larger interfragmentary axial torsion in the UTN group for the first five weeks post-operatively (Fig. 2, p ,< 0.035). In addition, anteroposterior and mediolateral bending angles showed significant differences between the groups for the first five weeks (p < 0.005 and p < 0.004) (Fig. 3). No statistical differences could be observed for the amounts of axial compression and shear movement (Table 1, Panel B). Interfragmentary movements decreased throughout the healing period. The axial torsion of the two groups converged at five weeks post-operatively (Fig. 2). In both groups, the sheep unloaded the operated hindlimb (Fig. 4) and a simultaneous overloading of the contralateral hindlimb was observed: Although both the contact force and the contact area of the operated limb decreased, the contact time of both hind limbs increased, demonstrating a reduced walking speed. The unloading of the affected limb was more pronounced in the UTN group (p < 0.009). Whilst the animals in the external fixator group returned to normal weight bearing within 60 days, no return to normal weight bearing could be observed in the UTN group within the nine week period. In fact, the unloading of the operated limb consistently increased until 35 days post-operatively in the UTN group (Fig. 4). Biomechanical testing of the callus postmortem showed significantly less torsional strength (p = 0.005) in combination with less torsional stiffness (p = 0.035) for
t
**
***
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**
**t
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tt**
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.E 3 U K a,
;
21-
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-External Fixator t u n r e a m e d Tibial Nail
37 u 0,
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20 30 40 days post operation
60
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Fig, 3. Mediolateral (top) and anteroposterior bending (bottom) at the center of the fracture site throughout the healing period (mll bending: p* = 0.001, p** = 0.004, p*** = 0.001, p"'* = 0.004; alp bending: p * = 0.005,p"=0.004,p**'=0.001,p***'=0.041).
lZ01
1
It**
6External Fixator
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l+Unreamed 0
10
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Tibial Nail 20
30
40
50
60
days post operation
Fig, 4. Maximum ground reaction force of the right hindlimb throughout the healing period, shown as a percentage of the preoperative values @" = 0.005, p * 2= 0.008, p*3= 0.009, p'4 = 0.004, p*' = 0.001, p*6= 0.004, p*' = 0.003). 0
10
20 30 40 days post operation
50
60
. ^. . . Fig. 2. Axial torsion throughout the healing period of the proximal in .
. ..
.
respect to the distal fragment is significantly smaller in the external fixator group @* = 0.035), p'* = 0.004, p***= 0.005, p*"* = 0.002).
the UTN group compared to the external fixator group: In the UTN group the torsional strength amounted to 50.3 k 16.5% of the intact contralateral side versus 78.5 k 11% in the E F group whilst the average torsional
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P. Klein et a1 I Journal of Orthopaedic Research 22 (2004) 1072-1078
stiffness amounted to 69.3+_27%in the UTN group versus 91.4k7.1 in the EF group. Histologically, the callus tissue appeared to be nonuniform in the UTN group, with osseous regions surrounded predominantly by fibrous tissue (Fig. 5 , top). Complete periosteal bone bridging of the gap could only be observed laterally, whilst on the medial side, bone bridging was incomplete. Osseous filling of the intercortical gap did not occur in the UTN group. In the external fixator group the periosteal callus was delimited, bulbous and pronounced on the medial side. In contrast to the UTN group where the nail filled mostly the medullary space, the intercortical gap was completely bridged with mineralized bone tissue in the external fixator group (Fig. 5 , bottom). The total width of the callus (p==0.002) as well as the mineralized bone width (p == 0.002) were significantly lower in the UTN group. In the same group, the thickness of the cortices was reduced, and significantly less cortical bone (p = 0.001) and cortical mineralized bone (p = 0.002) could be seen. In addition, the bone perimeter was significantly reduced (p == 0.002) when compared with the external fixator group.
Fig. 5. Comparison of histological results: Safranin-Orangehon Kossa staining of an osteotomy site nine weeks after unreamed nailing (top) and external fixation (bottom). The medial cortical bone is on the lefthand side of the images.
The in vitro biomechanical testing showed that the unreamed tibial nail provided significantly less torsional stiffness than the external fixator (Table 1, Panel A, p = 0.004). No statistical differences could be observed for the amounts of axial compression, whilst the anteroposterior and mediolateral shear stiffness’ (p = 0.026 and p = 0.009 respectively), and the anteroposterior and mediolateral bending stiffness’ differed significantly (p = 0.002 and p = 0.041 respectively) between the bone-implant constructs in vitro. Interindividual variations were generally larger in the UTN group in these in vitro tests.
Discussion The goal of this study was to determine the initial mechanical environment in a gap situation, simulating a not reduced fracture with unreamed tibial nailing compared to external fixation and to demonstrate the influence of the mechanical environment on the long term biological outcome in a sheep model in vivo. Comparison of two types of osteosyntheses in sheep has shown a greater unloading of the operated limb and a less developed callus formation in the animals treated with an unreamed tibial nail. The animals treated with the external fixator showed significantly less interfragmentary movement in the early stages of healing and returned to full weight bearing within 60 days postoperatively, suggesting near complete healing of the fracture. This observation was supported by the postmortem mechanical stiffness of the callus which returned to within 20% of the contralateral intact tibia. Loading of the operated limb in the UTN group, however, progressively decreased until week five, after which loading began to increase. At nine weeks post-operatively, the UTN group sheep were still not fully loading the operated limb, and the biomechanical as well as histomorphometrical differences in callus quality were still significant for healing. It is known that functional weight bearing is able to accelerate the rate of fracture healing and thus to significantly improve the strength of the healing bone [25]. However, early, full weight bearing with flexible fixation has shown delayed fracture healing [l]. In addition, it has been reported that the maximum displacement at the fracture site occurred when the affected limb was unloaded and that partial weight bearing, as described in clinical situations, does not necessarily lead to an unloading of the affected limb [10,24]. From this, it seems that the initial unloading of the limb may have had a long term influence on callus healing, possibly causing a slower healing rate compared to the fixator group. Since the initial stability of the osteosynthesis is a crucial factor for the success of treatment, the larger movements in the UTN group may have caused a
P. Klein et al. I Journal qf’Orthopuedic Research 22 (2004) 1072-1078
delayed callus formation compared to the fixator group [6,13,18]. The post-operative axial torsion, as well as the mediolateral and anteroposterior bending angles, were significantly larger in vivo within the first five weeks, most likely as a direct consequence of the different stiffness’ of the bone-implant constructs. Compared to other animal experimental analyses, the interfragmentary movements found for the fixator group in this study were in the described range which has been found to be optimal for fracture healing throughout the entire healing period. In contrast, in the UTN group the interfragmentary movements had decreased to within limits that are not deemed acceptable until nine weeks post-operation [5,13]. Histomorphometrical analysis showed obvious differences in morphology of the callus between the two groups. The periosteal callus showed considerable differences, also representing delayed callus formation for the unreamed nail group. These histomorphometrical findings were closely supported by the results of reduced biomechanical stiffness of the fracture callus in the UTN group. In addition, histology of the UTN group displayed signs of cortical thinning [3,1 I]. The bone loss of the diaphyseal cortex is thought to be a result of the rigid nail in combination with the large interfragmentary movements observed in this group, since rigid implants were described to lead to a necrosis of the cortical bone due to the stress to which it was exposed [3,8,29]. For the unreamed nail, the in vitro determined stiffness of the bone-implant constructs using sheep tibiae was comparable with those reported for human tibiae [27,28]. The poor healing observed for the UTN group in this study has not been reported for clinical situations so far. This might be attributed to differences in anatomy (e.g. absence of the fibula in sheep) and musculo-skeletal loading conditions that further determined the biological healing process [9]. Due to the experimental nature of both the defect situation as well as the osteosynthesis, the findings of the present study may not be able to be directly transferred into clinical situations. Nevertheless, some general conclusions should be drawn. In a clinical setting, it is well known that fracture gaps of more than 3 mm should be avoided in osteosyntheses with unreamed tibial nailing due to the risk of delayed healing (by 12 times) or non-unions (fourfold) [ 121. Therefore, this sheep model confirmed the recommendation of a precise reduction of the fracture fragments as a pre-requisite for good fracture healing. The data of the present study illustrate that otherwise the mechanical conditions in locked unreamed nailing may lead to delayed fracture healing. The unreamed tibial nailing could be further optimized by increasing the in vivo stability, particularly the axial torsional stiffness of the osteosynthesis. If it were possible to mimic the rotational stiffness of the presented external fixation in the bone-nail construct, then it may
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be possible to combine the advantages of the locked unreamed nailing with the rapid healing rates of the more stable osteosynthetic treatment options. This could help to avoid the severe mechanical conditions which seem to be detrimental to the process of bone healing.
Acknowledgements This study was partially supported by a grant of the A 0 Foundation Davos and the German Research Foundation (DFG K F G 102/1). The authors would like to thank Prof. S. Perren, Davos, Switzerland for editing this manuscript and Prof. Dr. M. Raschke, Muenster, Germany for clinical support.
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[24] Sarmiento A, McKellop HA, Llinas A, et al. Effect of loading and fracture motions on diaphyseal tibial fractures. J Orthop Res 1996;14:804. [25] Sarmiento A, Schaeffer JF, Beckerman L, Latta LL, Enis JE. Fracture healing in rat femora as affected by functional weightbearing. J Bone Joint Surg [Am] 1977;59:369-75. [26] Schandelmaier P, Krettek C, Rudolf J, Tscherne H. Outcome of tibial shaft fractures with severe soft tissue injury treated by unreamed nailing versus external fixation. J Trauma 1995;39:70711. [27] Schandelmaier P, Krettek C, Tscherne H. Biomechanical studies of 9 tibial interlocking nails in a bone-implant unit. Unfallchirurg 1994;97:60&8. [28] Schandelmaier P, Krettek C, Tscherne H. Biomechanical study of nine different tibia locking nails. J Orthop Trauma 1996;10:3744. [29] Uhthoff HK, Dubuc FL. Bone structure changes in the dog under rigid internal fixation. Clin Orthop 1971;81:165-70. [30] Williams EA, Rand JA, An KN, Chao EY, Kelly PJ. The early healing of tibial osteotomies stabilized by one-plane or two-plane external fixation. J Bone Joint Surg Am 1987;69:35545. [31] Wu JJ, Shyr HS, Chao EY, Kelly PJ. Comparison of osteotomy healing under external fixation devices with different stiffness characteristics. J Bone Joint Surg Am 1984;66:1258-64. [32] Yamagishi M, Yoshimura Y. The biomechanics of fracture healing. J Bone Joint Surg 1955;37-A:1035-68.