A non-localisable lameness can be one of the most frustrating problems in small animal orthopaedics. Localisation of pain and discomfort is essential in the work-up of lame dogs in order to identify cause, prognosis and treatment options. However, in some cases, it is difficult to determine the exact localisation of lameness because of the absence of palpable changes or because of unreliable pain responses. If pain is multifocal or impossible to elucidate on manipulation, skeletal survey radiographs may be indicated to localise the lameness. However, radiographic signs often lag behind ongoing skeletal pathology, underestimate soft tissue changes and may reflect previous disease processes.

While the use of expensive and invasive advanced techniques including arthroscopy, computed tomography (CT) and magnetic resonance imaging (MRI) have received considerable interest, when working up a non-localizable lameness it is advisable to employ minimally invasive diagnostic techniques first. The source of the lameness can be located using non-invasive techniques at least 80% of the time, even with challenging thoracic limb cases.¹

In cases where a full orthopaedic and neurological examination fails to convincingly locate the source of the lameness, three additional techniques may assist: arthrocentesis, intra-articular anaesthesia and nuclear scintigraphy.

Arthrocentesis

Laboratory evaluation can prove useful in lameness localisation where this remains elusive. When using arthrocentesis in the case of the non-localisable lameness, the first step in interpretation is deciding whether there is evidence of any pathology in the joint fluid; cell counts and cytologic examination are the most important components of analysis for this. Should there be evidence of pathology, the fluid can then become useful in differentiating between disease processes also. Based on cytology and fluid analysis, differentiation between inflammatory and non-inflammatory joint disease is possible. Through culture and sensitivity testing, differentiating between septic and non-septic inflammatory disease is possible.

Synovial fluid is normally clear and colourless to slightly yellow and is highly viscous; viscosity is lost with degradation or dilution of hyaluronic acid as occurs with joint inflammation. Significant haemarthrosis or xanthochromia is abnormal. Normal joint fluid contains less than 1500–3000 white blood cells/µL with a predominance of mononuclear cells (90%); neutrophils represent less than 5–10% of the normal cell count. Non-inflammatory joint conditions may be normal on fluid analysis or may result in nucleated cell counts up to 10,000/µL.² Most cells should be mononuclear although vacuolated changes in the mononuclear cells may increase (10%).²

A significantly elevated nucleated cell count in joint fluid is indicative of an inflammatory arthropathy. Nonerosive immune-mediated polyarthritis is the most common inflammatory joint disorder in dogs where white blood cells are typically elevated from 3000–100,000/µL.³ Neutrophils predominate and range from 30–100% of the cells seen.³ Mononuclear cells are often vacuolated.³ Cell counts in septic arthritis range from 40,000 to 100,000/µL or greater.⁴ Degenerative neutrophils and bacteria may or may not be identified on cytology.

Intra-articular Anaesthesia

In horses and humans, intra-articular administration of local anaesthetic is commonly used for diagnostic purposes because of the simplicity, safety and low cost of the technique and it can also be used in dogs. The principle is simple; local anaesthetic is injected into the joint, the anaesthetic is then absorbed and sensation in the area is temporarily blocked. Intra-articular anaesthesia can temporarily resolve lameness caused by a variety of lesions including synovitis, bone and cartilage fragments, intra-articular ligament damage and eroded cartilage.

Local anaesthetics are most commonly used for diagnostic intra-articular anaesthesia. Mepivicaine is the most commonly used local anaesthetic for diagnostic purposes in horses because it is less irritant and longer-lasting than lidocaine. In humans, bupivacaine was the most commonly used anaesthetic for diagnostic intra-articular anaesthesia for years. However, bupivacaine has a narrow therapeutic margin of safety and its use has largely been discontinued. Ropivacaine and levobupivacaine are more recently developed local amino-amides with a slow onset and long duration of action. These products are similar to bupivacaine and have been developed as safer alternatives. When carefully used at appropriate doses, local anaesthetics rarely induce harmful side effects; toxicity mainly results from accidental intravascular injection.

As with arthrocentesis, potentially any joint can be injected with local anaesthetic. An intra-articular injection can be painful; sedation is generally recommended in order to ameliorate this and facilitate accurate administration. Sedation itself can interfere with lameness evaluation, however, as sedative drugs may have an analgesic effect of their own. Careful selection of the sedative drug and dose is important. A study investigated the use of two sedation protocols for use before diagnostic intra-articular anaesthesia in lame dogs,⁵ comparing the use of acepromazine with methadone and medetomidine antagonized with atipamezole and their effects on lameness. The effect of these sedation protocols on the degree of lameness was negligible and should not interfere with the use of intra-articular anaesthesia to localise the source of lameness.

The area to be infiltrated should be clipped and cleaned with an appropriate antiseptic solution. The appropriate needle size and length depends on the size and depth of the joint. Synovial fluid is aspirated before injecting local anaesthetic to confirm intra-articular positioning of the needle. Care must be taken to avoid injection into a vascular structure in order to prevent systemic side effects; if blood is aspirated the needle should be redirected. The local anaesthetic dosage depends on the size of the patient and the size of the injected joint. It is important to apply the minimally effective volume of local anaesthetic. A large amount will enhance leakage of the local anaesthetic from the joint into the peri-articular tissue, resulting in a reduced precision in pain localisation, but too small a volume will lead to insufficient anaesthesia leading to a false negative response. In horses, adequate anaesthesia can be achieved in as little as one minute in some cases, but often 10–15 minutes are required before the anaesthesia takes full effect. It is therefore necessary to observe the patient over a sufficient period of time to assess the effect of the local anaesthetic.

Medial coronoid process disease can be a challenge to diagnose due to limited clinical and radiographic signs and a study investigated the use of diagnostic intra-articular anaesthesia of the elbow in dogs with suspected coronoid disease.⁶ Out of 90 dogs with confirmed disease, intra-articular anaesthesia resulted in an improvement in lameness in 87%, confirming the elbow joint as the source of lameness. This study confirmed that intra-articular anaesthesia can be a valuable diagnostic tool in dogs.

Nuclear Scintigraphy

Bone scintigraphy is an extremely sensitive tool for detecting changes in bone metabolism associated with skeletal diseases, injuries and arthropathies. Scintigraphy can also be of assistance in directing other investigations such as targeted CT examination and arthroscopy. Technetium-labelled diphosphonates are used as bone radionuclides. Following administration, technetium-labelled diphosphonates are incorporated into bone mineral and accurately reflect the status of bone remodeling as well as tissue perfusion. The radiopharmaceutical is injected intravenously and images can be acquired from two hours after injection.

Scintigraphy has been used for many years to localise equine lameness but its use in dogs in cases of occult lameness is less common. However, for canine lameness that cannot be diagnosed by conventional means, scintigraphy is a simple and useful procedure to rule in/out skeletal pathology.

Scintigraphy has been found to be helpful in dogs with non-localisable lameness in two studies.^7,8 Scintigraphy showed marked changes where clinical and radiographic signs were absent or equivocal and allowed lameness to be localised in the majority of cases. Bone scintigraphy is a technique with high sensitivity but low specificity and the exact nature of pathological uptake cannot be determined. Following lameness localisation, further structural imaging such as radiography, CT or arthroscopy will be indicated to further characterise the disease process.

Cases of occult lameness are not uncommon in dogs and can be a source of intense frustration. Arthrocentesis and intra-articular anaesthesia can be constructive diagnostic tools when faced with these cases and are easily performed in the primary care environment. In cases where the source of lameness remains unknown, nuclear scintigraphy may offer a cost-effective option for lameness localisation.

References

1.  Cook JL, Cook CR. Bilateral shoulder and elbow arthroscopy in dogs with forelimb lameness: diagnostic findings and treatment outcomes. Vet Surg. 2009;38:224–232.

2.  Lozier SM, Menard M. Arthrocentesis and synovial fluid analysis. In: Bojrab MJ, ed. Current Techniques in Small Animal Surgery. 4th ed. Baltimore, MD: Williams & Wilkins; 1998:1057.

3.  Hopper PE. Immune-mediated joint disease. In: Slatter DS, ed. Textbook of Small Animal Surgery. 2nd ed. Philadelphia, PA: WB Saunders; 1993:1928.

4.  Smith MM. Orthopedic infections. In: Slatter DS, ed. Textbook of Small Animal Surgery. 2nd ed. Philadelphia, PA: WB Saunders; 1993:1685.

5.  Van Vynckt D, Samoy Y, Polis I, et al. Evaluation of two sedation protocols for use before diagnostic intra-articular anaesthesia in lame dogs. J Small Anim Pract. 2011;52:638–644.

6.  Van Vynckt D, Verhoeven G, Saunders J, et al. Diagnostic intra-articular anaesthesia of the elbow in dogs with medial coronoid process disease. Vet Comp Orthop Traumatol. 2012;25:307–313.

7.  Schwarz T, Johnson VS, Vonte L, et al. Bone scintigraphy in the investigation of occult lameness in the dog. J Small Anim Pract. 2004;45:232–237.

8.  Samoy Y, Van Ryssen B, Van Calenberg A, et al. Single-phase bone scintigraphy in dogs with obscure lameness. J Small Anim Pract. 2008;49:444–450.