Introduction

Elbow dysplasia is an important cause of thoracic limb lameness in medium to large breed dogs. Elbow dysplasia includes three different pathologies: disease of the medial aspect of the coronoid process (MCD); osteochondrosis (OC) or osteochondritis dissecans (OCD) of the medial humeral condyle; ununited anconeal process (UAP). Elbow incongruity is likely a major contributory factor to most forms of elbow dysplasia and because of the potential multiple manifestations, developmental elbow disease (DED) may be a more appropriate generic umbrella term for the clinical syndrome.

Medial Coronoid Process Disease

MCD is by far the most common manifestation of DED. MCD is a more accurate descriptor than fragmented medial coronoid process (MCP) because up to 40% of all affected elbows have not actually fragmented.^1,2 The early stages of MCD attributed to physiologic overload of the MCP have been well documented revealing a pattern of subchondral trabecular disruption leading to diffuse microcrack formation and overt fragmentation.^3,4 Later changes associated with progressive MCD often culminate in progressive erosion of the cartilage of both the medial aspect of the humeral condyle and the MCP and it is generally accepted that this is because of humeroulnar conflict. As cartilage erosion continues, MCD plus disease of the medial aspect of the humeral condyle is referred to as medial compartment disease. It is now well established that the MCP can be affected by a vast spectrum of disease varying from superficial cartilage fibrillation to various patterns of fissure- fragmentation to full thickness cartilage erosion and subchondral bone disease.

There are several hypotheses regarding the etiopathogenesis of these pathologic changes, all of which may be attributed to some form of humeroulnar conflict. Hypotheses include:

 Static radioulnar length disparity

 Dynamic radioulnar longitudinal incongruency

 Incongruency associated with the shape of the ulnar trochlear notch

 Primary rotational instability of the radius and ulna relative to the distal humerus

 Musculotendinous mismatch

Identification of MCD has historically proven challenging despite the use of diverse imaging modalities including radiography, CT, scintigraphy and MRI. Typically, diagnosis is based on identification of secondary markers of osteoarthritis in the absence of other discernible discrete pathologic changes. Detailed radiographic examination is recommended before any intervention; this allows more detailed classification of the nature of the disease. The degree of osteophytosis has been shown to correlate with the severity of articular cartilage pathology and therefore may be relevant to decision-making.² CT and MRI scans are increasingly used to evaluate elbows where changes are equivocal on radiographic assessment. Cases with equivocal findings on both CT and MRI exist where subchondral bone pathology is later proven by histological examination of coronoid specimens after arthroscopically identified MCD.¹

Arthroscopic evaluation constitutes the most important single interrogation directing decision-making for MCD. If results of non-invasive imaging techniques are equivocal, direct observation by arthroscopy/arthrotomy may be justified.

Treatment options include:

 Nonsurgical management - when focal surgical treatment is inappropriate or did not resolve clinical signs

 Surgical management:

 Removal of free fragments alone

 Removal of free fragments with varying degrees of debridement of the visibly diseased portion of the MCP

 Subtotal coronoid ostectomy

 In cases with dynamic joint incongruency or abnormal dynamic loading as potential contributors to MCD corrective ulnar ostectomy may be required.

 In cases where rotational instability with excessive supination loading force is suspected, a biceps ulnar release procedure may be indicated.

The prognosis for cases of MCD is uncertain. Following initial therapy many dogs will demonstrate an improvement in lameness and they will often return to normal activity. They may need anti-inflammatory medications and/or rest for short bouts of lameness even in the short term. In the medium- to long-term, the prognosis is more guarded. Regardless of the treatment performed, osteoarthritis will progress with time and medical treatment may be needed on a more frequent or even continuous basis. For those patients where medical management is ineffective, further surgical options may be considered.

While arthroscopy has been reported to have several advantages over arthrotomy including reduced scarring, more rapid return to function, exceptional joint visualisation and less invasive technique, it also has some disadvantages. Arthroscopic equipment is expensive and effective use requires specialised training and experience. Relatively few primary care practices have access to CT or arthroscopy and therefore with owners for whom referral is not an option, arthrotomy may be the only option to obtain a definitive diagnosis and allow treatment. In one study comparing elbow arthroscopy to arthrotomy, it was concluded that postoperative morbidity should not be a factor when deciding which to use.⁵ Another study concluded that in cases treated by arthroscopy the period of convalescence was shorter and that better functional results were achieved following arthroscopy when compared to arthrotomy; however, the development of secondary arthrosis was not avoided by either technique.⁶ When performing an arthrotomy, the approach to the medial aspect of the humeral condyle and the MCP by an intermuscular incision⁷ is recommended.

Osteochondrosis of the Medial Humeral Condyle

OC and resultant OCD is a well-recognised disease of the medial compartment of the elbow and concomitance with MCD is frequent. This may reflect a potential role for incongruency in the pathogenesis of both conditions. Numerous other factors have been implicated in the pathogenesis of OC including genetics, diet and growth rate. The failure of chondral and subchondral blood supply is also widely accepted as being important.

Both surgical and nonsurgical management of OC of the medial humeral condyle result in progression of osteoarthritis, yet, variation in prognostic outcome within the spectrum of identified disease and detailed medium- and long-term outcomes have typically not been reported. Generally, the presence of marked cartilage disease of the medial humeral condyle is associated with relatively poor clinical outcomes and may progress to full thickness eburnation throughout the medial compartment.

Diagnosis of OC is possible in most cases on high quality, well-positioned radiographs with the craniocaudal view being most useful. The lesion is seen as an area of erosion or flattening in the subchondral bone. However, in some subtle cases, more advanced imaging in the form of CT, MRI or arthroscopy may be necessary.

Conventional surgical treatment of OCD lesions, including curettage, microfracture and micropicking, aim at stimulation of fibrocartilage ingrowth and are most appropriate for treatment of small lesions (typically <5 mm), shallow lesions (typically <1 mm) or abaxial lesions where the prognosis is anecdotally considered to be more positive. For more substantial lesions, regeneration of fibrocartilage has anecdotally been found to be inadequate for appropriate reconstruction of articular contour. Potential options for reconstruction of the articular contour include the osteochondral autograft transfer system (OATs) or the use of polyurethane "cartilage substitute" filler-plugs.

Ununited Anconeal Process

The anconeal process normally develops as an integral part of the ulna; however, in large and giant-breed dogs such as the German Shepherd, it develops as an ancillary ossification centre. In healthy puppies in which a separate ossification centre is present, fusion to the olecranon normally occurs by 20 weeks of age so a diagnosis of UAP should not be made until after this.

The proposed pathogenesis is that during growth, an overly long radius forces the humeral trochlea proximally against the anconeal process which results in joint incongruency, insult to the ossification centre and failure of the anconeal process to unite.

A flexed mediolateral radiograph is required for diagnosis. It is important to assess the radiograph for any evidence of incongruity, although this can be challenging unless incongruity is >2 mm. Failure to address incongruity surgically is likely to result in persistent lameness, hinder fusion of the UAP and promote osteoarthritis. UAP can present concurrently with MCD and, therefore, the medial compartment of the joint should be carefully evaluated.

UAP has been treated conventionally by three different procedures:

 Proximal ulnar osteotomy

 Anconeal process removal

 Anconeal process attachment ± proximal ulnar osteotomy

 Medical management has been shown to be inferior to surgical management and is associated with more rapid progression of osteoarthritis.⁸

Palliative/Salvage Procedures

Palliative procedures are undertaken when the joint disease is sufficiently severe that addressing the suspected underlying pathology and secondary sequelae is likely to be insufficient, or when more conservative management has failed. Procedures currently in this category are those that unload the medial joint compartment, replace joint surfaces, manage pain or remove the source of pain. These include:

 Sliding humeral osteotomy

 Proximal abducting ulnar osteotomy

 Canine unicompartmental elbow replacement

 Total elbow arthroplasty

 Elbow arthrodesis

 Elbow denervation

References

1.  Fitzpatrick N, Smith TJ, Evans RB, et al. Subtotal coronoid ostectomy for treatment of medial coronoid process disease in 263 dogs. Vet Surg. 2009;38:233–245.

2.  Farrell M, Heller J, Solano M, et al. Does radiographic arthrosis correlate with cartilage pathology in Labrador Retrievers affected by medial coronoid process disease? Vet Surg. 2014;43:155–165.

3.  Fitzpatrick N, Garcia TC, Daryani A, et al. Micro-CT structural analysis of the canine medial coronoid disease. Vet Surg. 2016;45:336–346.

4.  Danielson KC, Fitzpatrick N, Muir P, et al. Histomorphometry of fragmented medial coronoid process in dogs: a comparison of affected and normal coronoid processes. Vet Surg. 2006;35:501–509.

5.  Bubenik LJ, Johnson SA, Smith MM, et al. Evaluation of lameness associated with arthroscopy and arthrotomy of the normal canine cubital joint. Vet Surg. 2002;31:23–31.

6.  Meyer-Lindenberg A, Langhann A, Fehr M, et al. Arthrotomy versus arthroscopy in the treatment of the fragmented medial coronoid process of the ulna (FCP) in 421 dogs. Vet Comp Orthop Traumatol. 2003;16:204–210.

7.  Piermattei DL, Johnson KA. An Atlas of Surgical Approaches to the Bones and Joint of the Dog and Cat. 4th ed. Pennsylvania: Elsevier; 2004:226–231.

8.  Cross AR, Chambers JN. Ununited anconeal process of the canine elbow. Compend Contin Educ Pract Vet. 1997;19:349–361.