34th Annual Scientific Meeting proceedings

THR: New strategies for periprosthetic fractures

Periprosthetic fractures (PPFs) are fractures of the bones hosting a permanent implant such as around a total knee or a total hip replacement, for example. Specifically, fractures around a Total Hip Arthoplasty/ Replacement (THR) most commonly involve femoral fractures around the stem but, more rarely, can involve fractures around the acetabular component. PPFs are a very common reason for revision THR surgery in humans, especially in older patients, as age-related osteopenia and chronic bone loss produces long- term complications such as aseptic stem loosening and peri-implant fractures. Therefore, the incidence of femoral periprosthetic fractures increases with age and with time since the initial surgery, roughly at around 6 % incidence at 10 years post op and around 10 % at 20 years post op. In veterinary literature, these fractures always occur in the initial few days to weeks after the surgery, for uncemented systems, and months to years in cemented systems. The incidence of canine femoral PPFs varies with the study and the THR system used, but ranges from 1-13 %. The Vancouver classification, a human system, has been loosely adopted in the veterinary literature and is based on the location of the fracture, whether the associated stem is unstable or stable and whether there is peri implant bone loss. Type A fractures involve the greater / lesser trochanter, Type B occur around or just below the tip of the stem (B1, with a stable stem; B2, unstable stem; B3, loose stem with peri implant bone loss), Type C, fracture occurs well below the tip of the stem.  The precise mechanism for the pathogenesis of these fractures in dogs can perhaps be discussed in two scenarios. The proximal metaphyseal fractures, originating from fissures in the calcar area, can spiral down to involve the rest of the femur, allowing the stem to subside, rotate etc before osteointegration. The more distal fractures (Types B and C), with stable proximal metaphyseal sections and stable stems are perhaps more difficult to explain. The use of augmented BFX stems, such as collared and bolted stems, will be effective is reducing or eliminating the risk of early stem subsidence, which would help develop a subclinical proximal femoral fissure even if undetected in surgery. Applying a single proximal cerclage wire in the calcar area has also been shown effective to eliminate proximal femoral fractures in a large number of cases.   The increasing use of the lateral bolt system for BFX stem augmentation has likely changed the pattern of femoral PPFs in the dog since the literature seems to suggest that most of these fractures are Type B1 and Type C, in which the stem is stable within an intact proximal femoral segment.

Fracture repair in these cases typically involves very rigid internal fixation, reduction of these very typically oblique fractures with two or more single / double loop, heavy cerclage wire and the use of a lateral plate. A locking system is preferable, with a plate long enough to make the best use of the scantly available proximal bone of the greater trochanter and reaching the distal femoral diaphyseal-metaphyseal junction. Typically, three screws could be placed in the greater trochanter and three screws will be placed distal to the stem tip. Augmentation with additional proximal cerclage wires placed around the bone and over the plate will be important to reduce the shearing stress on the proximal screws, which are always short due to the lack of available bone purchase. The use of double / orthogonal plating, with a secondary plate placed on the cranial / craniolateral aspect of the femur and anchored in the available bone in the trochanter is an interesting modification in selected cases. Also, the use of cerclage buttons can help the application of this second plate, contributing to its fixation to the proximal femur in a more centered location. The presence of the stem negates the possibility of craniocaudally directed screws on a cranial plate and the cerclage buttons, placed on empty plate holes, can help anchor well-placed cerclage systems (loop wires or wire cables) around the proximal femur and both plates, and will avoid any potential loss of wire tension by distal slippage.

Biomedtrix: How to choose the right stem for every case

Biomedtrix offers a range of femoral stem for total hip replacement in dogs and cats and their indications and usages vary. This discussion will involve mainly the choice of uncemented stems since the decision- making process in cemented (CFX) application is largely or exclusively restricted to choosing the right size of stem. When it comes to uncemented (BFX) stems, the options are: the centerline stem, the standard BFX stem and the two augmented stems, the collared stem and the lateral bolt stem. The centerline stem, available since 2010, is indicated in cases with substantial femoral neck sclerosis or proximal femoral deformities that would prevent routine axial femoral canal preparation. It is the least commonly used variation, with a very small percentage of global use, compared to the other stems and, therefore, the lesser-known of the stems. The standard, collared and lateral bolt stem all share the same characteristics of general shape, material, osteointegration EBM surface dimensions and specifications etc. The general recommendations for the use of the collared stem is in patients with a canal flare index (CFI) of less than 1.8, especially in the presence of thin cortices but with proximal femoral cancellous bone of adequate strength. The lateral bolt stem uses a bolt which is locked into the neck of the stem and is introduced into the femur through a hole in the lateral cortex. This device effectively prevents any relevant subsidence or rotation of the stem before osteointegration occurs so the bolted stem is especially indicated in larger patients with weaker bone and femoral shapes less conducive to mechanical fit of the stem, i.e. more stove-pipe femur, with CFI less than 1.8. In essence, since the shape of these uncemented stems is identical, the mechanical interference of stem with the supporting cancellous and cortical bone of the proximal femur, vital to their initial stability until integration occurs, will depend of the size of the stem, the hardness of the bone and the shape of the femur. Choosing the shape of the uncemented stem from a large selection of available implants,  based on the geometry of an individual patient’s femur is not a luxury we have as veterinary surgeons.

The final choice of stem (CFX  vs BFX, which type of BFX…) is surgeon dependent. Some THR surgeons will hybridize the hip and choose to cement the stem in every case. Others will cement the stem only in cases in which the perceived or statistical risk of stem complications is higher (certain breed, advanced age, stove pipe femurs, thin cortices on radiographs, weak cancellous bone during canal preparation, etc).  In the pure BFX camp, there is also a variety of approaches, since some surgeons will use a standard BFX stem for every case, or a collared stem for every case etc, although some others will choose based on factors such as the ones just mentioned above.  Additionally, the surgeon who chooses a centerline stem for nearly every case does not have to factor in all the other considerations we have been discussing. There are no contraindications for the use of an augmented stem even if there is no absolute indication for its use, and no specific risk factors that would definitively justify it. Therefore, some surgeons feel that the use of a collared or, perhaps even more so, a bolted stem in every case affords an extra level of safety by reducing the chance of stem-related complications which justifies their extra cost. 

To this point, the global sales of the different stems has changed dramatically in the past few years with a dramatic increase in the use of bolted stems with a corresponding decrease of the standard BFX stem and the CFX stems. This is likely suggestive of a change in operative decisions by Biomedtrix surgeons, who now apply a bolted stem in cases that would have received a standard stem or perhaps a cemented stem in the past, due to a perceived increase in the risk of stem-related complications. This has certainly been the case of this speaker, who now only uses bolted BFX stems, in the sizes  for which it is available (from number 5 BFX stem up).

There is no definitive study that validates the Dorr classification for our patients, confirming the correlation between stovepipe femurs with weaker bone but the overwhelming clinical evidence is there. The use of the bolted BFX stem has allowed surgeons to apply this uncemented technology in cases that would have been previously considered only adequate for cemented stems.