Joint Medication: Part 2 - Commonly injected joints and is a joint block 'just' a joint block?

During this 4 part blog series, I am going to delve into joints, joint injections and other medications which can help with joint health.

This second part of the series will look into commonly injected joints, joint communication and to an extent, joint blocks. There are a significant amount of other joints, tendon sheaths and bursae which can also be injected with the drugs discussed in the third part of this blog series.

The first part of the blog looked at the composition of joints and development of osteoarthritis, the third will look at joint medications and the fourth will look at other medications and supplements which can be beneficial for joint help.

How is joint communication determined?

Some joints, like the hocks, caprus and stifle, are made out of multiple separate joints. These sometimes communicate, making interpretation of joint blocks tricky and result in joint medication affecting multiple compartments, which may not always be beneficial. Joint compartments can communicate in 2 ways:

1. Via an opening which is present from birth or due to trauma - this is a physical opening.

2. Without a physical opening, certain medications can pass through the joint barrier by a process called diffusion.

Physical communication is determined by injecting contrast material (dye) or medication into a specific joint. This can be done either with a live horse, or as an experiment on horses which have been put to sleep. The joint is then flexed and manipulated. This is imaged with radiographs, CT (computed tomography) or any other modality. This process is repeated a significant amount of times, to be able to determine how often the communication, or lack thereof, is present.

Obviously, most owners do not know the communication status of the joints of their horses. It is not overly important - however, understanding how a joint block and medication moves around joints is going to be important in comprehending why certain medications are more useful in specific joints.

How does a joint block work?

Joint blocks are an essential diagnostic tool and the gold standard in determining whether discomfort is coming from a specific joint. An anesthetic agent is injected into the joint after sterile preparation (trying to minimize the risk of infection) and the horse is assessed after a few minutes. This medication can sometimes pass through joint barriers. This is determined experimentally by obtaining a sample of joint fluid from a joint compartment which has not been injected, but suspected of potentially 'communicating' with the originally injected joint. This phenomenon of diffusion is also present with certain other drugs which are injected into joints.

Some horses obviously do not stand well for joint injections - sedation will delay the speed at which the horse can be trot up again, and the rest of the blog will show why this can, sometimes, result in false results. Other restraint techniques can include using a twitch, placing the horse in stocks, having a competent handler or picking up another leg.

Even then, some horses do not have a good temperament and cannot deal with injection into a joint. This can be exhibited by trying to kick the vet, risking injury and possibility of the needle snapping into the joint. Although this is extremely rare, it is an unfortunate risk of joint injections.

Hocks

The hocks correlate to our ankles. It is a complex joint, consisting of four principal levels of articulation. From top to bottom, these are the:

1. Tarsocrural: main hock joint where most movement occurs.

2. Proximal intertarsal (talocalcaneal– centroquartal): communicates with tarsocrural joint.

3. Distal intertarsal (centrodistal): communicates with tarsometatarsal in a minority of horse (8% to 39%)

4. Tarsometatarsal: communicated with the tarsocrural joint in only 3% of cases.

The joint which is most commonly blocked is the tarsometatarsal (TMT) joint. This is represented by the filled in star below. The distal intertarsal joint is represented by the empty star.

What else gets blocked when the TMT is blocked?

Within 15 minutes of injection, the local anesthetic spreads to the tarsocrural joint in approximately 90% of horses. This means that if the period between injection and assessment is 15 minutes long, due to sedation or other circumstances, you cannot be 100% sure that the block is confined to the TMT and not actually due to pain in the centrodistal and tarsocrural joint (1). Not only that, but the block can also desensitize a number of soft tissue structures, including the suspensory ligament, tibialis cranials, fibularis tertius, tarsal sheath and deep digital flexor tendon (2).

The close relationship between the suspensory ligament and hock can make differentiation between disease of the two quite tricky, and proximal flexions tests can be positive with both.

Stifle

The stifle is the equivalent to our knee. It consists of two joints - the femoropatellar (joint between the femur and knee cap) and the femorotibial joint (between the femur and tibia).

The femorotibial joint (FTJ) is divided into a medial and lateral compartment and they do not usually have a structural communication. The lateral compartment is about 50% larger than the medial one, with a joint volume of approximately 60 ml in the lateral FTJ and 40 ml in the medial FTJ (3). This is significantly larger than most other joints and important to know when considering the volume of local anesthetic or medication which needs to be injected into each joint.

Although the FTJs do not have a communication portal, blocking the medial compartment will generally result in blocking the lateral compartment and vice versa (1). Blocking either FTJ results in desensitization of the femoropatellar joint in 85-90% of horses (1).

Radiograph of the stifle showing the lateral FTJ (filled circle), medial FTJ (empty circle) and rudimentary fibula (double circle)

What does this mean?

2 main things:

1. To block the whole stifle, at least one of the FTJs and the femoropatellar joint need to be injected.

2. Due to the size of the stifle, a larger volume of medication may need to be infected into the joint.

Is the stifle block only a stifle block?

As with the hock block, a stifle block can also give false positives in some horses. Nerves which run close to the joint capsule provide sensory information to that back leg’s foot. This means that blocking the stifle can block out that hind foot. A stifle block must therefore be assessed relatively quickly as 33% of horses with hind foot pain improved by 50% within 30 minutes of a stifle block (4).

Sacroiliac

The sacroiliac joint (SIJ) is the articulation between the spine and pelvis. It also links both of the back legs. Subsequently, in my experience, orthopaedic issues in other parts of the body nearly always affect the SIJ. I often compare the SIJ to a hinge between the front and back end, helping to shift weight away from another issue.

This compensatory pain found in the SIJ is highlighted in the fact that only 32% of horses which block out to their SIJ have ultrasonographic findings (5) and that only 14.5% of horses with SIJ pain had no other concurrent lameness issues (5). This means that the SIJ can be sore without obvious pathological findings and that more often than not, SIJ pain is secondary to other issues, mainly front foot, back or lower hock/ suspensory pain (5).

Again, the gold standard of diagnosing SIJ pain involves injecting anesthetic into the joint. This can be done in 3 main ways:

1. Cranial approach under ultrasound guidance - this is the most specific approach due to visualization of the needle entering the joint, however, there is a low risk of ataxia/ wobbliness with minimal long term side effects.

2. Medial approach - this is done blindly, and apart from ataxia, has an increased risk of also injecting the lumbosacral joint.

3. Caudal approach - this is also done blindly, and apart from ataxia, has an increased risk of also blocking the sciatic nerve.

Both sacroiliac joints are generally medicated at the same time.

Ultrasound image showing the needle (star) being advanced into the sacroiliac space, thus ensuring correct placement. It is directed in a cranio-caudal direction, meaning that the needle is inserted in front of the sacroiliac joint and directed towards the tail.

Carpus

The carpus joint is commonly termed a horse’s knee, however, it correlates with our wrist. Apart from arthritic changes, ‘coronation injuries’ can also happen from stumbling onto their knees.

Like the hock, it is a complex joint, however, it consists of three principal levels of articulation. From top to bottom, these are the:

1. Antebrachiocarpal - usually independent from the other joints, however, can communicate with the carpal sheath which contains the digital flexors (deep and superficial).

2. Middle carpal.

3. Carpometacarpal joints - has caudal outpouchings that link with the origin of the suspensory ligament.

The star shows pathological change to the bone surface.

The middle carpal and carpometacarpal joints always communicate, and they rarely communicate with the carpal sheath. Even though no physical communication is present between the antebrachiocarpal and other joints, local anesthetic injected into either compartment generally blocks the other joints too (6).

Fetlock

The fetlock is a fairly straight forward joint, complicated only by the presence of sesamoid bones and a tendon sheath which together help to guide the flexor tendons further down the leg and provide an insertion point for the suspensory ligament.

Double hexagon = fetlock joint, Braided hexagon = sesamoid, Filled hexagon = distal end of the splint bone.

Although a successful fetlock block mainly indicates lameness associated with the fetlock joint, it can also result in resolution of lameness associated with some of the sesamoidean ligaments (ligaments which hold the sesamoids in place) (7).

Analgesia of the tendon sheath is only indicative of issues within the tendon sheath itself (8).

Hoof

The hoof encapsulates two hugely important (they are all important) synovial structures: the coffin joint and navicular bursa. Structural communication between the two is fairly uncommon, and present in 5.3% of cases, mainly when pathological lesions are present (9). A block of the coffin joint does, however, always eventually block out the navicular bursa, however, this is less likely, but still possible (44%), when the navicular bursa is blocked out (6), further indicating the importance of interpreting joint blocks quickly.

Note the proximity of the coffin joint (star) and navicular bursa (banana).

Conclusion

The aim of this part of the blog series was to show how joints communicate, both through openings and also without. This means that it can be difficult to be certain that the cause of lameness is coming from a specific location, but also that certain medications can spread between multiple areas, limiting local efficacy.

References

1. Gough, M., Munroe, G. and Mayhew, I., 2002. Diffusion of mepivacaine between adjacent synovial structures in the horse. Part 2: tarsus and stifle. Equine Veterinary Journal, 34(1), pp.85-90.

2. Dyson, S. and Romero, J., 1993. An investigation of injection techniques for local analgesia of the equine distal tarsus and proximal metatarsus. Equine Veterinary Journal, 25(1), pp.30-35.

3. Trumble, T., Stick, J., Arnoczky, S. and Rosenstein, D., 1994. Consideration of anatomic and radiographic features of the caudal pouches of the femorotibial joints of horses for the purpose of arthroscopy. American Journal of Veterinary Research, 55(12), pp.1682-9.

4. Radtke, A., Fortier, L., Regan, S., Kraus, S. and Delco, M., 2020. Intra‐articular anaesthesia of the equine stifle improves foot lameness. Equine Veterinary Journal, 52(2), pp.314-319.

5. Barstow, A. and Dyson, S., 2015. Clinical features and diagnosis of sacroiliac joint region pain in 296 horses: 2004-2014. Equine Veterinary Education, 27(12), pp.637-647.

6. Gough, M., Mayhew, I. and Munroe, G., 2002. Diffusion of mepivacaine between adjacent synovial structures in the horse. Part 1: forelimb foot and carpus. Equine Veterinary Journal, 34(1), pp.80-84.

7. Sampson, S., Schneider, R., Tucker, R., Gavin, P., Zubrod, C. and Ho, C., 2007. Magnetic resonance imaging features of oblique straight distal sesamoidian desmitis in 27 horses. Veterinary Radiology & Ultrasound, 48(4), pp.303-311.

8. Jordana, M., Martens, A., Duchateau, L., Haspeslagh, M., Vanderperren, K., Oosterlinck, M. and Pille, F., 2016. Diffusion of mepivacaine to adjacent synovial structures after intrasynovial analgesia of the digital flexor tendon sheath. Equine Veterinary Journal, 48(3), pp.326-330.

9. Hontoir, F., Rejas, E., Falticeanu, A., Nisolle, J., Simon, V., Nicaise, C., Clegg, P. and Vandeweerd, J., 2019. Communication between the distal interphalangeal joint and the navicular bursa in the horse at Computed Tomography Arthrography. Anatomia, Histologia, Embryologia, 48(2), pp.133-141.