34th Annual Scientific Meeting proceedings

Introduction
Treatment of HIF focuses on two main aims. The first is to reduce micromotion between the medial and lateral aspects of the humeral condyle and hereby decrease associated lameness, while the second is to decrease the risk of future humeral condylar fracture. The presence of a HIF has been shown to elevate maximum principal bone strain in the humeral condyle and lateral epicondylar crest (McCarthy et al 2024); resolution of the HIF is anticipated to reduce strain to more normal levels and ameliorate the increased risk of fracture. Traditionally, these aims have been achieved through placement of a transcondylar bone screw. The transcondylar screw acts to buttress the condyle, however, as the HIF is not expected to heal, such screws remain at risk of cyclic fatigue failure over time (Charles et al, 2009). It has therefore been recommended to use as large a cortical screw as possible.

A larger area moment of inertia is anticipated to reduce, or at least delay, the risk of future screw failure. The use of a screw with a thread diameter measuring 30-50% the width of the isthmus of the humeral condyle has been recommended as a guideline (Moores, 2021). Others have recommended placing a screw with an area moment of inertia of at least 0.3 times the body weight of the patient (Carwardine et al, 2024). However, even when these recommendations are followed, the risk for implant failure is not negated as is somewhat predictable when attempting to resolve an essentially biological deficit with a mechanical solution. In an attempt to reduce the incidence of screw fatigue failure, some surgeons advocate for the placement of locking screws, preferring these because of their increased core diameter. However, the rationale behind this is somewhat flawed.

Humeral transcondylar screws can be placed as either lag screws or position screws.  Definitive evidence showing that one method results in lower complication rates or preferential outcomes remains is not currently available. While one study reported a lower complication rate when lag screws were used (Hattersley et al, 2011), more recent studies report preferential results using position screws (Low et al, 2024 and Cawardine et al, 2024).

Original descriptions of transcondylar screw placement detailed placement of screws from lateral to medial. More recently, position screw placement in a medio-lateral orientation has been described and has been found to result in a lower complication rate (Clarke et al 2012, Carwardine et al 2024). While many surgeons have transitioned to using this technique, care is warranted when converting from lateral to medial placement as, with inexperience, there may be a greater risk of intra-articular screw placement (Barnes et al, 2014). Cannulated drill systems may be worth considering, at least early in the learning curve; in a case series of 13 cases operated via a medial approach using a cannulated drill system, none of the screws penetrated the articular surface (McCarthy et al 2020).  

The prognosis for resolution of lameness following transcondylar screw placement is excellent in most dogs.  Nevertheless, despite this being a straightforward surgical procedure, the risk of postoperative complications is consistently reported to be high, with rates varying from 17.5% to 69.2% (Low et al, 2024 and Chase et al, 2019). Complications are predominately accounted for by seroma formation and/or surgical site infection.  Less common complications include medial epicondylar fissure fracture, aseptic loosening and screw breakage. The reported incidence of screw breakage ranges from 2.5% to 10% and may be detected between 11 months and several years after surgery. While screw failure may result in recurrence of lameness or complete humeral condylar fracture, in other cases it may be clinically insignificant and the true incidence of screw failure following HIF treatment therefore remains unknown.

Non-Surgical Treatment
In asymptomatic cases of HIF, the decision may be made to manage these patients non-surgically. The risks of electing not to buttress the humeral condyle include the future development of lameness or condylar fracture; however, a study reported that these only occurred in only 18% and 6% of dogs respectively (Moores & Moores 2017). Given the aforementioned significant risk of postoperative complications, the respective risks of both treatment options should be weighed carefully.  

Alternative Methods and Insertion Techniques
In response to the persistently high complication rates reported with traditional techniques, various HIF-specific implant systems have been developed over the last two decades. Unfortunately, studies directly comparing these alternative implant systems to traditional cortical screws have not been performed. Findings for each system are limited to isolated case-series and the challenge to determine whether they present substantial advantages therefore remains.  

Alternative implant systems include Shaft screws, humeral intracondylar repair system (HIRS), humeral intracondylar stabilisation system (HISS) and Fitz Fenestrated Tubular Transcondylar (F²T²) screws with these systems employing increased AMI and/or biological augmentation to address concerns with simple cortical screws.

Transcondylar screws can be placed freehand, using custom 3D printed patient-specific guides, or with the use of commercially-available aiming devices, fluoroscopy, or arthroscopy.  Although the use of such aids has potential to improve the accuracy of transcondylar screw placement, their use does not necessarily reduce intra-operative or post-operative complication rates (Low et al, 2024). Indeed, fluoroscopically-guided transcondylar screw placement has been associated with relatively high intra-operative and post-operative complication rates of 6% and 45%, respectively (Carwardine et al, 2021). While no intraoperative complications were reported in one study where fluoroscopy and cannulated drill bits were used concomitantly, the postoperative complication rate remained high at 57% (McCarthy et al 2020).

More recent theories on the pathogenesis of HIF suggest humero-anconeal incongruity as a potential contributor (Danielski et al, 2022).  As a result, proximal ulnar osteotomy (PUO) has been suggested as a possible treatment option (Danielski et al, 2024) and one case report also reported the use of distal ulnar osteotomy (Alvarez and Motta 2024). It is theorised that PUO may ameliorate the abnormal load that is postulated to be applied to the humerus by the tip of the anconeal process, thereby allowing the HIF to fuse.  However, when PUO is performed for other conditions, such as medial coronoid disease, osteochondrosis of the medial humeral condyle and un-united anconeal process, it is associated with significant morbidity and prolonged recovery times.  Complications are reported in 12-54% of cases and include delayed or non-union, excessive callus formation, ulnar fissure fractures, pain, implant loosening/failure and infection. Therefore, treating HIF with a PUO as an alternative to transcondylar screw placement is controversial.

Skeletally Immature Dogs
Treatment of HIF/IOHC in skeletally immature dogs deserves specific consideration, particularly given recent evidence indicating variations in ossification times of the humeral condyle between breeds (Hutchings & Rutherford, 2024). Transcondylar screw placement has historically been recommended when HIF is diagnosed in skeletally immature dogs. However, two recent studies have demonstrated that with continued monitoring, fusion of HIF may occur in the absence of specific treatment (Garland et al, 2023 and Hutchings & Rutherford, 2024). Rather than immediately placing a transcondylar screw in these young HIF cases it may be more prudent to repeat imaging over a period of a further 6-12 weeks to ensure the HIF is not fusing, and to only consider transcondylar screw placement only in the presence of a persistent, unchanging fissure. 

Because of a lack of comparative studies, unanswered questions persist regarding ossification times and pathogenesis. In the absence of such studies, and those documenting number needed to treat, number needed to harm and absolute risk reduction for the various treatment modalities, treatment remains dependent on surgeon preference and experience. As such, it is essential that clients are fully informed about the respective benefits and risks of both stabilisation of the humeral condyle and of non-surgical treatment, so that they can engage in shared decision making appropriately.

References

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