Introduction

Joints, or articulations, form when two or more bones are united by a combination of fibrous, elastic, or cartilaginous tissue. They are divided into three main groups: fibrous, cartilaginous, and synovial joints. Synovial joints permit the greatest degree of movement and are the most commonly involved when considering diagnostic sampling. All synovial joints are characterized by a joint cavity, a joint capsule, synovial fluid, and articular cartilage.¹ The shoulder, elbow, carpus, hip, stifle, and tarsus are all synovial joints.

Arthrocentesis is the aspiration of synovial fluid from a joint space. Synovial fluid analysis is recommended in diagnostic workup for joint disease. It is useful for categorizing types of diseases, such as immune‐mediated, inflammatory, infectious, and occasionally, neoplastic. Synovial fluid analysis can be beneficial in diagnosing joint disease in a rapid manner based on cytology and cell count numbers. Osteoarthritis typically shows evidence of mild inflammatory change with mild to moderate increases in mononuclear cell numbers. Moderate to severe increases in mononuclear cell numbers and nondegenerative neutrophils is a more supportive cytology indicative of immune‐mediated polyarthritis; whereas degenerative neutrophils with or without organisms is indicative of infective arthritis.^2–4 The reader is directed to other texts for further detail regarding specific diseases of the joint and associated cytology and culture findings.^2,4,5

Arthrocentesis is a simple diagnostic procedure that carries minimal morbidity when performed appropriately. The procedure does not require extensive training or specialized equipment, lending itself to be a widely accessible diagnostic and therapeutic tool for the general veterinary clinician. Appropriate knowledge of pertinent anatomy and practice can quickly make arthrocentesis an invaluable tool for the diagnosis and treatment of a wide variety of joint diseases.⁶ This chapter is meant to provide the clinician with tools and information to become successful at performing arthrocentesis of various joints in the dog and cat.

Indications/Pre‐procedure Considerations

There are several indications for performing arthrocentesis and joint taps in veterinary patients, which can be categorized broadly into diagnostic and therapeutic reasons. Synovial fluid may be aspirated to obtain an initial diagnosis or to guide treatment for an ongoing disease process, such as septic arthritis or immune‐mediated polyarthritis. Therapeutic indications for placing a needle into the joint may include injecting medications, such as platelet‐rich plasma (PrP), or flushing joints to treat septic arthritis. Flushing the joint can also be utilized to check joint capsule integrity as part of a complete wound evaluation.

The clinician must remember to evaluate the patient as a whole prior to performing arthrocentesis. While most patients can tolerate arthrocentesis with sedation, the patient’s cardiovascular status and anesthetic candidacy must be evaluated to allow for a safe sedation or anesthetic event should general anesthesia be necessary. Smaller dogs and cats may require more extensive sedation or anesthesia to prevent iatrogenic hemorrhage from trauma to the synovium secondary to movement during the procedure.

Steps should be taken to ensure efficiency and decrease patient morbidity, including appropriate patient positioning and preparation. The clinician should decide how many joints need to be sampled and accordingly position the patient such that the least amount of repositioning is needed to access all joints. In cases where clearly one joint is affected, the clinician may elect to only sample said joint. If trying to diagnose immune‐mediated polyarthritis, multiple joints, preferably distal joints, such as a carpus and/or tarsus, should be sampled even if only one joint seems clinically affected.⁵ Anticipated tools needed for the procedure should be laid out for easy access, including but not limited to sterile needles (22‐gauge being the most commonly used), syringes, slides, gloves, culture tubes, and sample tubes (Figure 52.1).

Aseptic technique should be practiced when preparing the sites for arthrocentesis. The skin overlying the desired sample site should be evaluated for overt disease that would increase the chances of complication with this procedure. Although it has been demonstrated that effective aseptic skin preparation is possible without clipping short‐haired dogs, the authors still recommend that the fur over the joint of interest is clipped to allow sterile preparation and decrease the chance of iatrogenic introduction of joint infection.⁷

Surgical Procedure

Once positioned appropriately, the fur over the joints of interest is clipped, and the skin is aseptically prepared. Sterile gloves should be worn to facilitate palpation of the joint. It has been shown that larger syringes can generate more vacuum but result in relatively less needle control.⁸ Therefore, it is recommended to choose a syringe that is just large enough for a sample but not too small such that viscous joint fluid cannot be aspirated. This is most commonly a 3 mL syringe. Regular hypodermic needles have a relatively long bevel, which may preclude the lumen from entering the joint completely in smaller joints. In those cases, a spinal needle, which has a short bevel, may be of better yield.⁹

The clinician can decide whether or not to attach the needle to the syringe prior to or after the insertion of the needle into the joint. See below for techniques specific to each joint. Negative pressure is gently applied to the syringe, keeping in mind that normal joint fluid is viscous and requires more time to travel through the needle into the syringe. If no joint fluid is appreciated, sometimes rotating the needle on its axis and re‐aspirating may help dislodge any tissue that is blocking the lumen. Negative pressure should be released prior to redirecting and re‐aspirating the joint. Oftentimes, only a small amount of fluid, such as only enough to fill the needle hub, is required to achieve a diagnosis; the quality of the fluid is prioritized over the quantity.¹⁰ After synovial fluid has successfully been sampled, negative pressure is released, and the syringe and needle are removed from the joint. Alternatively, in efforts to prevent blood contamination, the clinician can firmly hold the needle hub in place while disconnecting the syringe. The needle is then removed.

The gross appearance of the synovial sample can often give the clinician a hint as to whether pathology is present or not in the joint. Normal joint fluid should be quite viscous, clear, and straw‐colored (Figure 52.2). Synovial fluid is a thixotropic fluid; it is less viscous when shaken, and will return to the original viscosity at rest.¹⁰ It should not be mistaken for a clot.

Samples from arthrocentesis should be immediately transferred to the medium of choice, whether it be for cytology or for culture. If there is blood contamination, the fluid may be placed in an EDTA tube to prevent clotting. However, EDTA will interfere with the mucin clot test¹ and culture results.¹⁰ To preserve cell morphology, direct smear slides should be prepared right after obtaining the sample. The clinician should keep in mind that because of the viscous nature of the joint fluid, the spreader slide should be moved slowly and evenly.¹¹ If the clinician wishes to culture the synovial fluid, the sample should be kept in the sterile syringe, placed in an aerobic culturette, or placed in a blood culture tube. Aerobic culturette tubes should be placed in the refrigerator and shipped on ice packs.¹² It has been shown in human prosthetic joint infections that blood culture tubes have higher sensitivity, specificity, and positive and negative predictive values when compared with standard tissue and swab samples.¹³ Additionally, blood cultures were shown to have improved detection rates in human septic arthritis as compared to traditional agar plates.¹⁴ Although it has been shown that a slight delay in sample incubation is not detrimental to the results,^15,16 it is still strongly recommended to incubate the blood culture tubes as soon as possible. The blood culture tubes should never be refrigerated or frozen.¹⁷

Shoulder

The shoulder joint is a ball‐and‐socket, diarthrodial joint that involves the articulation of the glenoid cavity of the scapula with the head of the humerus.¹⁸ It is a highly moveable joint, allowing flexion, extension, abduction, and adduction. The main movement of the shoulder is flexion and extension.¹⁹ The bony anatomy of the joint consists of the distal extremity of the scapula and the proximal humerus. Along the lateral surface of the shoulder is the spine of the scapula, which starts from the middle of the dorsal border and extends distally to the glenoid end, which forms a pointed structure known as the acromion process. The lateral surface of the humerus is the deltoid tuberosity and the brachial groove. An important feature to note at the proximal humerus is the crest of the greater tubercle. The craniolateral bony projection on the lateral surface of the proximal humerus is the greater tubercle, whereas the smaller bony prominence at the medial aspect is the lesser tubercle.¹⁸

The primary ligaments of the glenohumeral joint are the joint capsule and the two glenohumeral ligaments. The joint capsule attaches proximally at the periphery of the labrum, which is the glenoid lip that surrounds the glenoid. The joint capsule attaches distally to the distal portion of the humeral head. The glenohumeral ligaments, both lateral and medial, arise from the supraglenoid tubercle and end caudal to the lesser tubercle of the humerus. These collateral ligaments bridge the joint on the lateral and medial aspects.^{18, 20–22}

There are multiple muscles, nerves, and vessels also comprising the anatomy of the shoulder joint. The tendon of the biceps brachii muscle, supraspinatus and infraspinatus muscles, subscapularis muscle, omotransversarius and deltoideus muscles, and tendons of the teres major and minor muscles are all involved in the shoulder joint anatomy. The biceps brachii muscle originates from the supraglenoid tubercle of the scapular bone, crosses the shoulder joint, and inserts on the cranial surface of the humerus to the joint capsule at the intertubercular groove.^18,21,23 It is at the intertubercular groove where the joint capsule surrounds the tendon of the biceps brachii muscle, and it is held in place by the transverse humeral retinaculum. The axillary, subscapularis, brachial and radial nerves, and arteries are located cranial to the shoulder joint, while the subscapularis artery and nerve are located caudally.^18,20,21

Knowledge of the anatomy of the shoulder joint is vital prior to performing arthrocentesis to minimize complications and contamination. For arthrocentesis of the shoulder joint, it is necessary for the patient to be sedated. The sedated patient is placed in lateral recumbency. Important landmarks to palpate prior to arthrocentesis are the greater tubercle and acromion (Figure 52.3). In general, the joint is located distal to the acromion and proximal to the greater tubercle. Using sterile technique, a 22‐gauge needle attached to a 3cc syringe, is inserted and directed craniolateral to caudomedial, entering the joint between the greater tubercle and the acromion, shown in Figure 52.4.^24–26 A longer needle may be required for large dogs. Once the needle is advanced into the joint, synovial fluid can be aspirated. Synovial fluid analysis is critical in diagnosing immune‐mediated and septic disease of the shoulder joint; however, these diseases are less common in the shoulder joint. It is more common for shoulder joint disease to be consistent with degenerative joint disease, in which case synovial analysis can be normal despite intra‐articular disease. If degenerative joint disease is suspected in the joint, further diagnostics, such as advanced imaging (radiographs, musculoskeletal ultrasound, or computed tomography) is recommended in addition to synovial analysis.^24,25

Elbow

The elbow is a hinge joint with a range of motion of 36° in flexion and 165° in extension. Motion in torsion and mediolateral plane is limited due to the anconeal process and collateral ligaments,^27,28 which make it a “ginglymus” joint. The medial collateral ligament attaches proximally to the medial epicondyle of the humerus and then divides into cranial and caudal crura. It attaches distally to the radius and ulna. The lateral collateral ligament is stronger than the medial, attaches proximally to the lateral epicondyle of the humerus, and divides in a similar manner to the medial collateral ligament. The annular ligament is located deep in the collateral ligaments and prevents cranial translation of the radius by its attachments on the proximal radius and ulna. The ligaments are external to the joint capsule. The joint capsule encloses the joint and forms two compartments: cranial and caudal. An inner synovial layer and an outer fibrous layer comprise the joint capsule.^27,29,30

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