Palmar/ Plantar Tenosynovitis
A tendon is a band of fibrous connective tissue that links muscle to bone. Unlike ligaments (which function to prevent excessive movement of structures relative to one another), tendons are used to produce movement. As the horse's muscle contracts, tension along its corresponding tendon increases, thereby resulting in movement of the bone(s) to which it is attached.
Learn more about the difference between tendons and ligaments HERE.
There are two basic types of tendons: FLEXOR TENDONS and EXTENSOR TENDONS.
Flexor tendons are designed to flex joints via the relative movement of bones on either side of the articulation. They typically course along the backside of the limb.
Extensor tendons function to extend (or straighten) joints via the relative movement of bones on either side of the articulation. They are usually located along the frontside of the limb.
Tendons must traverse at least one articulation (joint surface) in order to perform their function and require lubrication along each angled (or "bent") juncture to allow for smooth and effortless movement. Without sufficient lubrication, the tendons would wear excessively due to the increased friction they would encounter as their fiber orientation changed while coursing along the curved surface(s).
The tendon sheath is a synovial "sleeve" that houses the tendon(s) as they course along joint surfaces and provides the free-flowing environment necessary for normal function. It is comprised of thin connective tissue with an internal synovial lining, which is responsible for producing the lubricating (synovial) fluid that protects the tendons as they glide across angled joint surfaces. The fluid within tendon sheaths is identical to that found in other synovial structures such as joints and bursas.
Inflammation within a tendon sheath is termed TENOSYNOVITIS.
The three most commonly-affected sheaths in the horse all provide lubrication to the superficial and deep digital flexor tendons:
Digital tendon sheath, which houses the superficial and the deep digital flexor tendons as they course along the back-side of the fetlock joint. The digital sheath extends from just above the fetlock joint to the level of the proximal interphalangeal (or pastern) joint. The horse has four digital sheaths, two thoracic (or palmar) digital sheaths and two pelvic (or plantar) digital sheaths. These structures are often collectively referred to as PD sheaths.
Carpal tendon sheath, which encompasses the superficial and deep digital flexor tendons along the back of the carpus (or knee). This sheath extends from approximately 10-15cm above the carpus to as much as 25-30cm below the carpus.
Tarsal tendon sheath, which wraps around the superficial and deep digital flexor tendons near the back of the hock (tarsus) beginning at the top of the calcaneus (at the level of the tarsocrural joint) to a few inches below the tarsometatarsal joint.
This article will discuss the basics of PD tenosynovitis, which is the most common form of the three.
Tenosynovitis usually occurs as a result of one or more of the following:
Tenopathy. Inflamed, torn, and/or fibrotic (scarred) tendons often result in PD tenosynovitis. Damage to the tendon(s) can occur from internal or external injury.
Compromise of the sheath. The sheath (or sleeve) lining can also tear, become fibrotic and/or be a source of irritation within the synovial structure.
The introduction of an irritating matter (such as bacteria or a foreign body) into the sheath via laceration or puncture. This may result in septic tenosynovitis, which usually requires treatment in the form of surgical debridement and aggressive antimicrobial therapy.
External trauma. The sheath resides just under the skin along the distal aspect (bottom) of the horse's limb, and is therefore somewhat susceptible to external injury.
Constriction of the Annular Ligament. Demonstrable constriction of the flexor tendons within
the palmar digital sheath as a consequence of fibrosis (or scarring) and thickening of the annular ligament may be implicated as a cause of PD tenosynovitis.
Click HERE to learn more about annular ligament constriction in the horse.
The most obvious manifestation of digital tenosynovitis is visible distention (or effusion) of the proximal or upper portion of the sheath, which is evident along the back of the limb just above the fetlock joint. Swelling of this nature is often classified as a windpuff. In many cases, careful inspection reveals concurrent swelling associated with the distal or lower portion of the sheath along the back of the pastern region.
It should be noted that the term windpuff is not always emblematic of PD sheath swelling, but rather is a general term designating non-specific swelling around the fetlock area of the horse. Effusion (synovial swelling) of the fetlock joint, for example, may also be categorized as a "windpuff", even though it is suggestive of an entirely different issue.
Extra synovial fluid within the sheath suggests the presence of inflammation.
Pain Upon Palpation
Although somewhat crude in nature, a lot can be gleaned through thorough digital palpation of the region. In most cases, problems associated with the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT) and/or sheath lining can be discerned via this method.
Cytological analysis of synovial fluid aspirated from a tendon sheath can provide insight with regard to the nature of possible problems. In cases in which infection is suspected, supplemental culture of the fluid may also be performed.
The use of ultrasound allows for the visualization of all structures associated with the PD sheath, and is therefore an integral component of the diagnostic process.
Pathologic lesions associated with the flexor tendon(s) and sheath lining are easily discernible through ultrasonographic visualization. Dynamic studies are often performed to idenitfy potential adhesions between the sheath and tendon and/or between multiple tendons.
Tenoscopy involves the insertion of an endoscope (identical to that used for arthroscopy) into the tendon sheath.
This method provides a minimally-invasive approach that allows for direct visual examination and maximum exposure of the sheath's interior contents. Surgical debridement of the sheath as well as reparative procedures are also possible under tenoscopic guidance (see below).
Frequently diagnosed problems resulting in tenosynovitis include tendon compromise/ injury, tearing of the sheath capsule/ lining, the development of adhesions between the sheath lining and adjacent tendon fibers, and constriction of the annular ligament. These issues are easily idenitified using the aforementioned imaging modalities.
Of course, supplemental use of local (intrathecal) anesthesia can assist in confirming PD tenosynovitis as a source of pain and lameness when necessary.
Regardless of the cause of tenosynovitis, we must reestablish normal synovial integrity and associated structure function if we are going maintain a sound horse.
We have the following 3 goals with regard to successful treatment:
1) Alleviate excessive flexor tendon tension.
This is particularly important with respect to the digital flexor tendon sheaths. Excessive tension of the flexor tendons is most typically experienced by horses that exhibit a broken-back distal limb axis. We can approximate the angle-of-derangement with the naked eye or more easily using lateral radiographic images of the distal limb.
The presence of PD tenosynovitis may suggest excessive flexor tendon tension. Other clinical manifestations of excessive flexor tendon tension may include upright distal limb (foot) angle, contracted heels, dishing and/or ridging of the hoof wall, delamination of the hoof wall, and/or prolapse of the sole.
Reducing flexor tendon tension is most easily accomplished via corrective trimming and shoeing. Our shoeing recommendations are designed to reduce flexor tendon tension both statically (at a standstill) and dynamically (while moving).
We often recommend the following:
Trimming as much solar toe relative to heel as possible to increase the angle of the foot.
Applying wedged pads as necessary to maintain a straight (or slightly broken-forward) distal limb axis.
Shoeing to the toe to maintain weightbearing stress along the dorsal hoof wall.
Rockering the toe aggressively to facilitate breakover.
Rockering the toe just lateral (outside) to center to accommodate the horse's typical pattern of breakover.
We would be happy to provide you with a written Farrier Prescription at your request.
2) Alleviate inflammation and pain within the sheath.
Reducing inflammation decreases the amount of fibrosis (scar tissue) that will eventually develop. With less scar tissue restricting movement of the sheath and associated flexor tendons we can expect better eventual functionality and performance.
In the acute phase, we typically recommend topical therapy in the form of ice, cold-water hosing, Dexamethasone Sweat Spray and/or Diclofenac Sodium (Surpass®) in combination with systemic antiinflammatory therapy (e.g. Phenylbutazone) and stall rest. Our focus is to "quiet down" the acute inflammatory reaction as quickly and effectively as possible. On average, this takes about 10-14 days.
Once the initial inflammation has subsided, we often elect to perform intrathecal injection. By reversing the inflammatory process within the tendon sheath, we can also reverse the deleterious consequences that go along with inflammation. Moreover, we can effectively reestablish a normal synovial environment via the stimulation of hyaluronan production and release by the sheath's synovial membrane.
Intrathecal injection is best performed at the onset of the chronic stage (immediately pursuant to the acute phase). At this point, very little (if any) irreversible changes have occurred.
In most cases a combination of synthetic hyaluronan and steroid is infused into the sheath using aseptic technique. We implement the same medications as those used in intraarticular (joint) injections.
Click HERE to learn more about intraarticular medications.
3) Increase "pliability" to the sheath lining.
Over time scar tissue developing within the compromised portions of the damaged sheath matures and organizes. During this phase of healing the scar tissue contracts thereby restricting the movement of the sheath. In severe cases, the presence of chronic inflammation can promote the development of adhesions between the sheath lining and associated flexor tendons. In both cases, the sheath lining is more predisposed to future reinjury and associated lameness is more likely to develop.
In our experience, extracorporeal shock wave therapy (ESWT) has proved to be extremely beneficial in counteracting the "shrinking" effects of scar tissue that develops as the sheath heals. ESWT has also proved to be useful in preventing and breaking down intrasynovial adhesions that often occur pursuant to sheath injuries as well as promoting the disintegration of pathologic ossifications within the sheath lining and/or tendon structure(s).
Surgical intervention is indicated in some cases of intrasynovial foreign bodies, tendon rupture, septic tenosynovitis, adhesion formation and annular ligament constriction. It should be noted that surgical manipulation itself is considered a common source of eventual sheath fibrosis and adhesion formation. Surgical cases, therefore, should be selected cautiously.
Tenoscopy is far more advantageous than tenotomy (fully opening the tendon sheath surgically) for the following reasons:
Visualization of the internal sheath and associated structures is far superior using tenoscopy.
Recovery time is much quicker as a result of smaller surgical incisions required for tenoscopy.
There is less chance of postoperative complications such as infection and adhesions following tenoscopic surgical exploration/ manipulation.
Early return to exercise following tenoscopy is desirable to prevent adhesion formation.
Prognosis for future performance following surgical manipulation is generally good although will vary depending on the nature and severity of the lesion(s).