Introduction

Interlocking nail (ILN) has been in use since the 1950's. The veterinary use began in the 1990's, after Durall & Diaz-Bertrana (1996) have published results obtained with their models. A number of complex fractures with contraindication for a single Steinmann pin have been currently treated with locking screws, in a dynamic or static way, inserted into transcortical holes (Johnson et al. 1984). Intramedullary pins provide good resistance to bending, but do not have adequate stability against axial and rotational forces. Failures caused by material fatigue in absence of locking screws are rarely mentioned (Larin et al. 2001). In the dynamics of interlocking nail, the screw placement into their holes determines the stress concentration in both the proximal and the distal fragment (Bucholz et al. 1987). Bucholz et al. (1987) and Larin et al. (2001) recommend that ILN should reach the cancellous bone at the distal epiphysis minimizing the load on other parts of the implant such as the locking screws. Also, return of limb function should take place only after evidence of consolidation. However, in 2008, Romano et al. presented good results using nailing that did not reach the cancellous bone of the distal femoral epiphysis in feline with early support. Durall et al. (2003) developed a multiple-hole nail model, of a single length, to adjust the nail to the length of the medullary canal of the fractured bone, after previous cut using appropriate instrument. The cut, made in the hole region, shows irregular surface in the distal end of the nail. Their work first reported in veterinary medicine a circular lesion around the distal end of the nail similar to the one known in medicine as windshield-wiper effect (Ebraheim et al., 2002). Durall et al. (1993), Durall & Diaz-Bertrana (1996) and Dueland et al. (1999) argue that when ILN is applied to the femur, it should not be deeply inserted into the distal cancellous bone, due to the natural curvature of the canine femur, as it will determine the cranial angulation of the distal fragment caused by the natural curvature, resulting in mal-union. Schmaedecke (2007) studied the resistance of this implant against bending, compression and rotation forces, and also reported the possibility of using nails occupying the entire length of the medullary canal and little shorter nails, so far taken as unsuitable. Giordano (2004) developed the first Brazilian ILN, using 6mm and 8mm nails and 2.7 and 3.5mm locking screws respectively. The nails had a distal chamfer designed for better adaptation to the cancellous. Schmaedecke (2005) observed that during the insertion of these nails, they should not be directed cranially to the distal femoral fragment. Some surgeons choose bolts (Court-Brown 2003). Bolts are less susceptible to breaking, as they do not have screw in the region of contact with ILN, providing therefore larger area for load distribution and smaller area of stress concentration, with less possibility of crack nucleation (Callister Jr. 2005). In nearly two decades of study, a lot has been developed and published on the ILN use in veterinary medicine, but much remains to be investigated. This study aimed to present the changes in the distal third of the femur associated with the use of a modified ILN, showing a chamfer at the distal end, in contact or not with the cancellous bone of the distal femur.

Materials and Methods

The study methodology was subjected to the Ethics Committee for Research on Animals of the Department of Veterinary Medicine, at the Federal University of Viçosa (UFV), in August 2008 and approved with the opinion number 25/2008. We selected 15 female adult dogs, mixed breed, weight between 6.8 and 14.4 kilograms from the experimental kennel of the UFV Veterinary Hospital (HOV). At the selection time, radiographs of the animals' pelvic limbs were obtained, which were overlapped on a template of an ILN kit assisting in the determination of diameters and lengths of orthopedic nail and screws to be used in each animal. In addition, diaphysis femoral allografts preserved in glycerin 98% from the bone bank at the HOV-UFV were selected, adapted to 3cm in length, to be implanted in bone defects created in the middle third of the femur through surgery, after two transverse osteotomies. An ILN instrumental kit for small animals, up to 15kg, manufactured and traded in Brazil was used in the surgery. The animals were subjected to clinical evaluation during the experimental period and the surgical wound was examined daily until healing. Analysis of limb function was carried out every 24 hours as well as radiographic monitoring immediately after surgery and at intervals of 15 days until 120 days of the postoperative period.

Results

Of the 15 animals, 8 showed complications in the distal end, and of these, 6 showed area of circular and progressive osteolysis in the nail cancellous bone interface, portion in contact with the nail, compatible with the windshield-wiper effect (Figures 1, 2 and 3). Of these 6 animals, 2 presented spontaneous cure of this effect, going through a period of increased mineral deposition in response to damage caused by the ILN instability (Figure 1). Six animals had the radiographic findings of this effect accompanied by clinical signs and was linked to excessive instability caused by the breaking of the locking screws (Figures 2 and 3). In 2 animals, in which ILN has not reached the distal cancellous bone, there was slight cranial direction, with an area of resorption of cranial portion of the femoral cortical and periosteal reaction, overloading the distal locking screws and finally breaking by fatigue. Only after the breaking of screws, the animals showed clinical signs such as lameness, pain on limb palpation and bone crepitus on physical examination. Radiographic examination showed worsening of both the periosteal reaction and resorption of cranial portion of the femoral cortical (Figure 3).

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Figure 1. A) Mediolateral radiograph of distal femur, increase in mineral deposition (gray arrow) and B) Craniocaudal radiograph of distal femur: observe the periosteal reaction (white arrow) and increase in mineral deposition in area of lesion caused by ILNs instability / spontaneous healing of the windshield-wiper effect after 60 days (gray arrow).

Figure 2. A) Mediolateral radiograph of distal femur and periosteal reaction (white arrows) and B) Craniocaudal radiograph of distal femur: windshield-wiper effect after 30 days (gray arrow) post-breakage of the distal screw in the distal fragment (dashed arrow), observe the osteogenesis surrounding this screw (white arrows).

Figure 3. A) Mediolateral radiograph of the distal femur; observe breaking of the 2 locking screws (dashed arrows) and windshield-wiper effect (gray arrow), B) Craniocaudal radiograph of distal femur: windshield-wiper effect (gray arrow) , misalignment between the allograft and distal fragment (pointed arrows) and intense periosteal reaction at 28 days (white arrows).

Figures 4. A) Scheme of the forces acting at the time of impaction of the pin until it finds the cancellous bone of distal femoral epiphysis: direction and sense of the applied force when impacting the pin in the medullary canal (gray arrow); direction and sense of the force applied by the nail to the cancellous bone (dashed arrow); direction and sense of the bone reaction force on the nail (pointed arrow), resulting force and caudal displacement of the nail and consequent misalignment in relation to external drill guide (white arrow), B) Cranial displacement of the external guide in relation to the guide and consequently to the femoral anatomic axis.

Discussion and Conclusions

The windshield-wiper effect observed was similar to that described by Durall et al. (2003) and Durall et al. (2004), who used cuttable ILN of irregular distal end applied in different settings. Similarly, in 2 animals of this experiment these osteolysis areas were probably filled after fracture stabilization, finding, in the lysis region, increased mineral deposition during healing, in response to the lesion caused by the ILN instability. In 2 other animals this radiographic finding was accompanied by clinical signs such as lameness, pain on limb palpation and crackles on physical examination, which lasted until new surgery. These findings were related to excessive instability caused by the break of the locking screws, similar to those reported by Durall et al. (2004) after removing the fixing pins of the external holder in the distal fragment, aiming at energizing the system. In the two cases, both by the break of screws and the programmed removal of the fixing pins of the external holder attached to the pin, the result was similar, i.e., mobility of the ILN end leading to the windshield-wiper effect. The other animals showing the windshield-wiper effect had no clinical signs and presented limb function within 24 hours postoperative (Figures 1 A and B). There are no reports in the veterinary literature on complications involving the ILN distal end developed by Dueland, using trocar point. Romano et al. (2008) showed good results with this kit on feline even when it has not achieved the spongy bone of the distal epiphysis. Even if there was perhaps overload of locking screws, fatigue failures were not found in this study. Moreover, Schmaedecke (2007) found in in vitro biomechanical tests no significant difference between the groups using ILNs of suitable length, 185mm, and ILNs of unsuitable length, 160mm, whose nail did not reach the cancellous bone of the distal femoral epiphysis, but his study did not aim at results of cyclic loading tests. Whereas in this study, the results found in 2 dogs whose ILNs have not reached the distal cancellous bone, showed overload on the distal locking screws followed by break by fatigue (Figures 2 and 3), corroborating with the findings suggested by Bucholz et al. (1987) and obtained with the cuttable nails by Durall et al. (2003). We should also point out that the dogs used in this experiment were heavier, so the cyclic loading on the screws of this experiment was also higher than the weights of the animals used by Romano et al. (2008). Giordano (2004), also working with a model showing distal chamfer, has not found the windshield-wiper effect, however, he was using 6mm and 8mm nails, even if they were inserted into the distal epiphysis of the femur. This finding indicates that the used nail thickness may be a determinant for nail mobility at the distal femoral end. There was a slight cranial direction in the 2 dogs. It was observed periosteal and reabsorptive reaction at this point of the cortical, being recommended care to avoid it in the trans-operatory as suggested by Schmaedecke et al. (2005) and Schmaedecke (2007). Besides, the chamfer was considered a complicating factor for the insertion of the nail into the cancellous bone of the distal epiphysis, since when contacting it, if there is more impactation as it happens with the Steinmann pin, the nail will apply force to the cancellous bone, which in reaction it will apply force of equal intensity and opposite direction, resulting in caudal displacement of the nail in the medullary canal, thus misaligning the external drill jig in relation to the nail, leading to failure in distal locking (Figures 3 and 4).

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