**5.5 Excessive stiffness of implants**

The concept that stiffer implants delay bone healing assumes that a callus cannot be formed when strain conditions are too low. In a situation where strain is 0%, potentially this could delay healing. However, it is an unlikely situation in clinical scenarios. When animals use the limb, the amount of force applied always causes a strain value at the fracture site over 0%, and for that reason, there will be an unrealistic complication in small animals. Under optimal stiffness repairs are much more common in veterinary patients, often delayed, and nonunions are a consequence of inadequate addressing of fracture mechanics and/or poor biology versus too stiff implants. Low-strain environments created by stiffer implants facilitate haversian canals and faster bone regeneration.

#### **5.6 The concept of elastic osteosynthesis**

This concept is very specific to juvenile dogs and cats. Different breeds of dogs reach skeletal maturity at different ages; it is considered that the physiological process is finished between 5 months (toy breeds) and 18 months (giant breeds) through a very rapid and biphasic growth rate. During the initial growth phase, both structural and material properties of immature bone are considerably different from those of adult bone and are characterized by lower strength and stiffness, as well as lower yield stress and elastic modulus. Additionally, the diaphyseal cortices are thinner but have a more robust periosteum in young animals compared to in adults. As a consequence, immature canine bone is more predisposed to implant failure due to screw pull-out. In addition, due to the rapid initial growth phase and the natural flexion angle of the elbow and knee, postoperative immobilization of these joints in young dogs will inevitably lead to ankylosis secondary to adhesion formation and muscle contracture. In the hind limb, if the functional recovery does not happen early on after osteosynthesis, fracture disease leads to irreversible loss of function due to muscle contracture even after a few days of immobilization. To prevent this debilitating complication, early osteosynthesis is recommended to promote controlled postoperative mobilization, which can lead to implant failure due to the hyperactive nature of non-leash-trained puppies.

The use of overly rigid fixation in juveniles can lead to concentrated forces at the screw-bone interface. In the situation of a standard cortical screw, in this poor-quality soft juvenile bone, this could result in poor screw purchase, screw loosening, and subsequent implant failure, mostly due to screw pull-out. This situation is less common with locking screws, as for a locking screw to fail, it needs to cut through the bone.

Regardless of the osteosynthesis technique chosen and used in juvenile or pediatric dogs, physes must be preserved at all cost. This absolute requirement contraindicates the use of any intramedullary implants (e.g., pins or interlocking nails) especially during the first, rapid growing phase where the physes are more sensitive to traumatic closure. The external fixation is not the technique of first choice for the osteosynthesis of humeral or femoral diaphyseal fractures in young dogs due to mechanical and biological reasons. Namely, the outward position of the external fixator construct, away from the neutral axis of the bone, elevates the bending stresses at the pin/bone interface, promoting a stress riser point. These osteosynthesis technique is also prone to early failure due to implant pull-out, and the use of positive profile transfixation pins does not reduce this complication. From a biological standpoint, the transfixation of the thigh or arm musculature increases the exudation at pin/soft-tissue interface

due to excessive movement and generates postoperative pain avoiding free range of motion (ROM) at the knee or elbow. The resulting loss of ROM potentially leads to muscle contracture. Due to the potential complications associated with intramedullary pining and external fixation techniques, plate osteosynthesis remains the treatment of first choice for diaphyseal fractures in juvenile dogs However, if the AO principles of anatomical reduction and rigid internal fixation were used routinely in early growth phase, it can result in catastrophic implant failure via screw pull-out, which leads to the creation of elastic plate osteosynthesis technique (EPO). The technique relies on the increased overall compliance of the bone/plate construct to reduce the risk of focal failure of the screw/bone interface. EPO is used in conjunction with minimally invasive surgical strategies (MIS) favoring restoration of alignment rather than anatomical reconstruction and percutaneous sliding plate techniques to further decrease postoperative morbidity and stimulate early functional recovery. The plates used in EPO were mainly veterinary cuttable plates preferably with locking screws used in a bridging function without anatomical reduction and hematoma disturbance due to their favorable effects on indirect bone healing. Indirect fracture reduction is accomplished by traction on the distal fragment with small fragment forceps and/or using the plate. Large fragments or an oblique fracture should be reduced with the aid of pointed reduction forceps but without attempting a precise reduction. Since anatomical reduction is not attempted; restoration of the bone length is achieved by determining the appropriate plate length from radiographic views of the contralateral intact bone. Since the fracture site is not exposed, it is beneficial to verify proper alignment via intraoperative radiography or fluoroscopy.

The plate is cut to the desired length according to the anticipated position of the screws relative to the growth plates and inserted epiperiosteal through two proximal and distal small incisions. Cortical screws are placed in the two most proximal and the two most distal holes of the plates without tapping to increase bone adherence. In order to decrease pull-out complication, the screws axis should always be oriented in diverging planes in relation to bone longitudinal axis.

The preservation of the strong periosteal sleeve, and the use of an undersized implant such as veterinary cuttable plates (VCP), allow controlled motion at the fracture site, which in turn promotes secondary bone healing via fast callus formation. The flexural or bending deformation of the bone/plate construct is controlled, in part, by the working length of the plate dimension. EPO guidelines recommended that the central plate span without screws should be as long as possible and include no less than 3 consecutive empty screw holes to increase compliance and reduce stress riser effect. This pattern of screw distribution increases the working length of the plate and therefore its compliance. As a result, it decreases the stress riser effect of a single empty screw hole, thus reducing the risk of implant fatigue failure. Furthermore, the enhanced compliance of the bone/plate system lowers the stress on the interface between the bone and screw, thus decreasing the possibility of screw pull-out. Another strategy to decrease screw pull-out complication would be the use cancellous screws instead of cortical screws. The cancellous screw has larger threads and a higher pitch as compared to the cortical screw, which makes its use indicated in metaphyseal bone, osteoporotic bone, or low-porosity bone as found in young patients.

The use of minimally invasive (percutaneous) plate osteosynthesis in conjunction with EPO further reduces postoperative morbidity and promotes early use of the fractured limb and a rapid functional recovery. With this method of osteosynthesis, bone union was achieved as early as two weeks and in all cases at four weeks postsurgically [27]. Surgical complications related to implant failures, such as screw pull-out

and plate plastic deformation, were not reported. Radiographically, callus remodeling could be visualized two months postoperatively, and the bony union was completed in four months [27]. Diaphyseal growth occurred without complications, and angular deformation was not observed in either epiphysis.

Although weight-bearing and ROM are recommended immediately after surgery, high-impact activities (jumping and rough play), while difficult to be truly controlled, should be avoided. In contrast, controlled physical activities such as leash walking, trotting, and swimming are beneficial to bone regeneration and should be stimulated.

#### **5.7 Osteosynthesis in toy-breed dogs**

In toy dog breeds, complications related to osteosynthesis were more frequently reported than in the general population [28]. Delayed or nonunion and stress protection have been documented in long bone fractures of toy breeds as the most frequent complications, with a special focus on the radius and ulna [29]. Refracture after plate removal is a common complication after stabilization of the radius and ulna fractures. Patient factors such as poor intraosseous vascularity and limited periosseous soft tissue coverage predispose small-breed dogs to healing complications [30].

Biological osteosynthesis techniques decreasing iatrogenic surgical trauma while yielding appropriate construct stability would appear to be advantageous for facilitating the healing of these fractures. External skeletal fixation can be used in toy-breed dogs; however, the radius is a very narrow bone, in addition to its elliptical cross section, which makes the placement of transosseous ESF pins technically challenging [28]. Piras et al. reported the use of circular external skeletal fixators (CESF) in radius and ulna fractures in 16 toy-breed dogs, all of which achieved union despite reporting a 40% minor complication rate, including pin and wire tract discharge [31]. Plate osteosynthesis classically is considered a successful surgical option despite the report of major complications in 18% of cases in one study [32]. Nevertheless, more recently published studies have described a reduction in complications overall or implantrelated. Hamilton et al. reported a series of 14 toy-breed dogs treated with a T-plate, all of which healed uneventfully [33]. Regarding function assessment, it was graded as excellent in six dogs, good in four, and fair in two dogs. Vallefuoco et al. only reported 9% of implant-related complications with the use of LCP plates, which could explain the lowering of complications over time [34]. Despite MIPO being recommended in this group of dogs due to poor intraosseous vascularity and limited periosseous soft tissue coverage, recent studies have shown that conventional plate fixation of these fractures is not associated with such a high complication rate when fractures are treated with an appropriately sized bone plate. Pozzi et al. reported a retrospective study that radius and ulna fractures managed with MIPO had similar alignment, reduction, and time to union as fractures managed with ORIF [29]. Arburn et al. also reported a low rate of complications (3%) when ORIF for distal radial fractures was used [35].

In toy breeds, any implant has the potential to lead to stress protection, which can cause osteopenia, especially in radius and ulna fractures. This does not mean that the use of flexible implants is an absolute indication for toy breeds. For the same reasons as above, plates without the appropriate stiffness will fail in the same way as for any dog, especially if the anatomical reduction is required and the fracture line is not uniformly compressed, leaving the transcortices without contact subjecting the plate to bending stress and more prone to fatigue failure.

Excessively rigid plate fixation has historically been considered to be associated with stress protection and subsequent osteopenia, which may in part be responsible for increased refracture risk in these breeds [36, 37]. Osteopenia induced by stressprotection has been reported as a frequent (7.1–20%) complication after plate osteosynthesis of distal radial and ulnar fractures in miniature and toy-breed dogs [28, 32, 38]. A low incidence (1.5%; 1/65) of osteopenia was reported in the study published by Aikawa et al., in part because of the selection of appropriate plate size and type (DCP vs. LCP) with a proper technique [39]. Stress protection-induced osteopenia can only be detected by long-term plate application follow-up [37]; therefore, long-term annual radiographic evaluations are needed to diagnose this complication. On the other hand, a recent study has assigned vascular compromise of the bone cortex as the main cause of osteopenia [40]. Stress protection may not be the cause of osteopenia in distal radial and ulnar fractures, and routine plate removal is not necessary when fractures, provided that plates of appropriate size and type are used and soft-tissue handling atraumatic not overlooked [28, 32]. The diameter of the screws used is another factor to be considered. If they occupy more than 30% of the width of the bone radius (as the maximum size allowed), the bone may have reduced bone strength or have impaired vascular supply, and this can be a reason for osteopenia development [41].

Implant-induced osteoporosis (IIO) or osteopenia can be caused by osteonecrosis of the bone occurring just below the plate that causes cortical bone thinning of about 40%, occurring at 24 weeks after dynamic compression by plate placement [42].

IIO is evolved by biphasic changes and is attributed to inadequate blood supply at 8–12 weeks and reduced mechanical bone stress at 24–36 weeks [37]. IIO is a relatively common complication in small dogs, caused by a process of insufficiently developed bone microvessels, after internal fixation with a conventional plate [43].

LCP plates are reported to preserve blood flow to the periosteum and enable angularly stable fixation, leading to increasingly used in small animal orthopedic surgery [44–46]. In contrast to DCP/LC-DCP in which stability is provided by frictional forces between the plate and bone, locking plates allow the plate to be placed away from the periosteal surface and do not require compression of the periosteum, preserving periosteal blood flow and achieving secondary bone healing due to relative stability [46]. Preserving periosteal blood flow during fracture treatment is an important factor for bone regeneration; as long as the blood flow is preserved, the risk of infection and IIO is reduced. LCP plates due to reportedly small periosteal contact areas reduce the risk of early postoperative osteoporosis and should be the main option for distal radial fractures in toy breeds [47].

Regarding the material used for plating, the comparative studies for the most common alloys used (titanium vs. stainless steel) did not show different results regarding stress shielding [48, 49]. However, titanium alloys produced more flexible plates compatible with the modulus of elasticity of bone. This flexibility is inductive of fracture healing in areas where higher strain values are needed to promote bone regeneration. Additionally, titanium alloy is reported to be more resistant to cyclic load and notch sensitivity when compared to stainless steel and from a theoretical point of view should be the first-choice material for implants used in this type of breed [50].

Plate removal is indicated if osteopenia or IIO is diagnosed due to the predisposition to refractures after implant removal. This procedure should be staged in two to three surgical procedures [51].

### **5.8 Minimally invasive plate osteosynthesis**

Minimally invasive plate osteosynthesis (MIPO) is a surgical approach to fracture treatment using bone plates, following principles that include (1) the use of indirect, closed reduction techniques; (2) epiperiosteal plate insertion through small incisions remote to the unexposed fracture site; and (3) minimal reliance on secondary implants and bone grafts [52].

This surgical approach emphasizes soft tissue preservation over anatomic reconstruction/absolute mechanical stability and is specially indicated for low-strain fractures. In most fractures repaired by MIPO techniques, the bone heals in conditions of relative stability. Relative stability relies on the use of implants that provide flexible fixation, allowing an acceptable degree of strain compatible (<2%) with bone regeneration. Osteosynthesis methods that are commonly used in MIPO are plates or platepin combinations applied in bridging function to span a bone defect not anatomically reduced, resulting in a relatively stable environment.

This technique is applicable in the treatment of most diaphyseal, metaphyseal, and periarticular fractures. The use of an intramedullary pin, particularly recommended in comminuted diaphyseal and metaphyseal fractures, is beneficial in facilitating the reduction and restoration of alignment [53]. The minimum recommended diameter for the IM pin is 30% of the medullary canal diameter at the bone isthmus [54].

MIPO is a surgical approach that often ends up with a long working length plate; however, this is because we have chosen to sacrifice the mechanics of our implant, to preserve the biology. This approach can favor the biological factors of bone regeneration, but the increased working length decreases the stiffness of the construct and therefore the fatigue life of the plate. The primary factors affecting the stiffness of the plate are the modulus of the material used, the AMI of the construct, and the working length. The factors influencing gap strain are gap width and the magnitude of motion between the fragments. Fatigue failure are determined by factors such as the yield bending strength of the construct and the cumulative load/number of cycles that are suffered by the plate. The rationale of the MIPO approach is to improve biological factors at the fracture site to speed up healing, preventing plates from prematurely failing due to fatigue failure.

In MIPO, the plate is applied as a bridging function; for that reason, the selection of an implant of appropriate length is a crucial step. With this surgical approach, it is recommended to use longer plates as possible for improving screw-working leverage and to distribute bending forces well along the plate, thereby lowering pull-out forces on screws. If the surgeon chooses the MIPO approach, selecting the adequate plate length in preoperative planning is a crucial step for bridging osteosynthesis. Two parameters are used to determine the plate length: the plate span ratio and the plate screw density. The plate span ratio is the quotient of plate length and segmental length of fractured/comminuted bone. The plate screw density is the quotient of the number of screws inserted and the number of screw holes available. For comminuted fractures, which are commonly treated with MIPO and bridging osteosynthesis, the plate span ratio should be greater than two to three. For simple fractures, this ratio ranges between eight and ten. In comminuted fractures, the plate working length may not be the distance between the screws closest to the fracture, but rather the unsupported area of the plate corresponding to the length of the fracture gap.

Plate screw density or screw-hole-ratio should be smaller than 0.5–0.4 in comminuted fractures and at least two to three screw holes empty over the bone defect [55]. For simple fractures, a value of 0.4–0.3 is recommended. Because this ratio is usually

applied to the whole plate, it may not be as applicable for highly comminuted fractures in shorter animal bones. Also, the screw density can be different in different bone segments due to the diversity of lengths, being higher in shorter segments and lower in longer segments. Mechanically, there was a poor advantage of adding more than 4 screws per fragment. Within a fragment, the guidelines advise placing 1 screw close (near) to the fracture and 1 at the very end of the plate (far) and then a minimum of 2 additional screws evenly spaced over the remaining span. Adding more screws offers no mechanical security but does add surgical damage to the bone [2].

The recommended ratio of plate length to bone length [Plate-Bridging Density (PBD)] should be less or equal to 0.91 0.05 [56].

Beyond location, the number of monocortical and bicortical screws in the construct is also influential on its biomechanical properties. Less torsional stiffness is provided with monocortical screws compared to with bicortical screws. When using LCP, a minimum of one screw must be placed bicortically in each major bone fragment due to a significantly increased torsional stability, based on the scientific evidence of a biomechanical study using bone models [18, 57].

Additionally, long plates enable plate insertion incisions to be created far from the fracture site. Surgical planning should include the exact location and sequencing of insertion of the screws to be placed. It is recommended to start inserting the first screw distally to center the plate in the distal segment. To align the bone and stabilize the fracture, the most proximal screw is next inserted into the proximal fracture segment. Additional screws are inserted and used to reduce the bone to the plate. When using a pre-contoured locking plate, it is recommended that a cortical screw be placed in both the distal and the proximal bone segments to frame the bone to the plate, further aligning the bone in the sagittal plane. After stabilizing the fracture with the 2 non-locking screws, locking screws are sequentially placed in the aforementioned order. Preoperative bone plate contouring is advisable to decrease surgical time with the MIPO technique. Preoperative plate contouring can be performed using contralateral bone radiographs or 3D printing models if the contralateral bone is not fractured [58].

An important factor to be considered is the alignment between the bone axis and the plate. Due to poor visualization of the bone surface caused by a limited surgical approach, malalignment between the bone axis and plate leads to an eccentric plate position can occur. At the proximal or distal end of the plate, a monocortical screw will not anchor in the bone [57]. To overcome this problem of insufficient anchorage of a monocortical self-drilling screw, a long bicortical self-tapping screw can be inserted or a standard screw allowing angulation in the plate hole [57]. However, this procedure can also cause iatrogenic fractures [59].

Dynamic compression plate (DCP), limited contact-dynamic compression plate (LC-DCP), or locking compression plate (LCP) systems have been used with success for MIPO procedures. Nowadays, the MIPO technique is almost performed in the majority of cases using a locking plate-screw interface, such as the LCP, due to the angular stability provided by this system, which by definition increases the loadcarrying ability of the construct. The angular stability originates from the threaded screw heads being locked into the threaded plate holes, thus forming a fixed-angle construct. Another important advantage of locking plates for use in MIPO is the minimal contouring needed for the application of the plate in contrast to DCP or LC-DCP, which requires optimal contouring to maintain the reduction of the fracture. Locking plates are considered internal fixators and therefore do not displace the fracture segments during screw tightening regardless of the precision of contouring.

The major disadvantages of using monoaxial locking implants are the inability to vary the angle of screw insertion through the hole (unless using a polyaxial locking plate system) and the increased cost of locking implants compared with that of standard plates and screws [58].

On the other hand, non-locking bone plates for MIPO offer other advantages, in radius and ulna fractures where the plate can be used to reduce and align the fracture segments in the sagittal plane. The relatively flat cranial surface of the radius allows precise reduction of the proximal and distal fracture segments as long as the plate has been preoperatively contoured. Many locking plates also allow the insertion of nonlocking (cortical) screws into the plate holes (combi holes). If a locking plate is precontoured and initially applied to the bone using a cortical screw in the proximal and distal fracture segment, then the locking plate can be used to align the radius in the sagittal plane similar to a non-locking plate. Once sagittal plane alignment is achieved, the remaining screws inserted should be locking screws, to take advantage of the angular stability provided. The cortical screws that were initially inserted may be left in place or replaced by locking screws [58].

LCPs also have the advantage of preserving periosteal vessels. The periosteal blood supply beneath locking plates is not damaged because compression between the plate and the bone does not occur because is not a plate-bone friction base system which improve and hastens bone healing and simultaneously reduced the risk of cortical bone necrosis and infection. Malunion or delayed union are infrequent complications when using this type of implant in MIPO. Regarding infection rates, when MIPO and ORIF are compared, there is a lack of evidence in veterinary studies, but in the human side, evidence showed lower infection rates when MIPO techniques are used in long bone fractures [29, 60–62].

Further advancements with intraoperative imaging such as fluoroscopy have the following aims: maximized biology due to a more limited surgical approach allows placing implants with a longer working length and improve alignment. Alignment of the main bone segments and the articular surfaces without torsional and angular deformities is also one of the main objectives of MIPO. Intraoperatory fracture alignment can be assessed by two methods: intraoperative diagnostic imaging and clinical evaluation. Intraoperative imaging is not always available in clinical practice, and for that reason, precise perioperative planning is a critical point for bone alignment in MIPO.

Fracture reduction under the plate (FRUP) is a technique that was developed by Cabassu et al. to improve bone alignment on MIPO without intraoperative imaging but requires precise preoperative contouring of the plate and extensive preoperative planning [63].

With the FRUP, the first step of surgical planning is to obtain radiographs of the fractured and contralateral bones, under sedation or general anesthesia. Two orthogonal projections of contralateral bone digital radiographs were obtained using a radiopaque marker (of known dimensions) to calibrate images for plate contouring. The choice of the type of fixation is based on fracture location/classification and biological and clinical factors. After calibrating the radiological image, the craniocaudal or mediolateral image of the long bone is used to contour the plate. Ideally, the plate length is selected to span from the proximal to the distal metaphysis of the bone when possible or based on a plate length/fracture length ratio of 3 (MIPO guidelines) [57]. The placement of the plate on the digital radiograph is oriented by anatomical landmarks that could be externally identified intraoperatively such as patella, medial tibial malleolus, femoral greater trochanter, ulna styloid process, lateral epicondyle, and

greater tubercle of the humerus. The plate is then anatomically contoured to adapt to the bone surface (e.g., the lateral face of the femur diaphysis and the medial surface of the tibial diaphysis). Fracture line(s) is drawn on the intact bone, which allows planning the number and the position of the screws to be inserted. First, the site to place the screws near the fracture is chosen. According to the MIPO guidelines, at least three empty screw holes should be respected over the fracture site [57].

When managing long oblique or comminuted fractures that have a significant gap, it is recommended to place one screw in each fragment as close to the fracture as possible. In comminuted fractures with a smaller gap, screws are placed with a minimum of three holes'space between them. In the outermost plate holes, one screw is placed in each proximal and distal fragment. Depending on the case, a third screw may be inserted between the inner and outermost screws [63]. The location for each screw is predetermined and identified by its hole number from proximal to distal. The type of screw, whether locking or cortical, is then selected. At least one cortical screw is used on the distal and proximal fragments to allow for fracture reduction, and these screws are placed first in these bone segments [63]. These screws are inserted in the diaphyseal segment of the bone in diaphyseal fractures or close to the fracture site in metaphyseal fractures. Afterward, the surgeon will then subjectively decide whether to place locking or cortical screws based on the screw location and angulation relative to the joint. The order of screw insertion is then selected, starting with the cortical screws used to reduce the fracture. Generally, the first screw inserted in the femur is in the proximal segment, while on the tibia, it is in the distal segment. The reason is that plate location was easiest to determine on these fragments. The cortical screws that were initially inserted may be left in place or removed and replaced by locking screws. Screw length is measured during preoperative planning as well as screw angulation (this is possible using a variable angle locking plate system) to avoid articular penetration. The plate is then sterilized the day before surgery or during patient preparation and draping. Specially designed "L," "Y," or "T" plates have proven to be very useful for MIPO stabilization of distal diaphyseal or metaphyseal fractures of the long bones (especially in radius fractures), which would normally be difficult to stabilize using straight plates [58].

Two skin incisions are made to the level of the bone surface away from the fracture, and an epiperiosteal tunnel is created, and the plate is slid onto the bone surface [64]. Anatomical references are identified, flowing by the alignment of one bone segment with the plate using bone-holding forceps; immediately after this step, the plate is fixed to the bone fragment using the first cortical screw, which is inserted perpendicular to the bone surface [63]. The opposite bone fragment is then distracted using bone-holding forceps to gain length, and alignment in torsional and axial planes, and temporarily stabilized to the plate to maintain alignment and length. Anatomical landmarks on the opposite fragment relative to the plate are checked, and the second cortical screw is inserted to obtain a reduction under the plate. The second cortical screw is then inserted to obtain a reduction under the plate. Alignment is assessed intraoperatively by evaluating the range of motion and alignment of adjacent joints in axial and frontal planes. When an intramedullary pin is used, the fracture is temporarily aligned under the plate and stabilized using bone forceps only to facilitate the intramedullary pin insertion. The pin is then inserted, and correct insertion is assessed by releasing the distal fragment from the plate. If the placement of the pin is evaluated as incorrect, the pin is removed from the distal fragment, and the fragment is manually mobilized to allow placement of the pin in the distal medullary cavity. The plate is then fixed in the same way as without using an intramedullary pin, and other screws
