Joints: Part 1
The mechanical engineering involved in the structuring of equine joints is both complex and masterful. Not only do healthy joints allow the horse to move freely, but they also help to effectively absorb concussion, especially when the horse is traveling at speed. In this article and one to follow next month, we want to take an in-depth look at joints and some of the problems that afflict them. Before we can discuss joint diseases, fractures, and other problems, we first must understand how joints are constructed and how they function. That will be the thrust of this article.
The information that follows has been gleaned from experts in the field, such as C. Wayne McIlwraith, BVSc, PhD, Diplomate ACVS, of Colorado State University; the late and legendary O. R. Adams, DVM, MS, also of Colorado State, who authored the book, Lameness In Horses; as well as Robert A. Kainer, DVM, MSD, and Thomas O. McCracken, MS, both of Colorado State.
To begin, there are three different types or classifications of joints--fibrous, cartilaginous, and synovial. Simply put, the purpose of a joint is to allow the back to flex and the limbs to bend.
Fibrous joints are basically immovable and united by fibrous tissue that ossifies with age. Included in this category are most joints of the skull (suture) and those between the shafts of some long bones (syndesmosis).
Cartilaginous joints have limited movement, such as the pelvis and vertebrae, and the growth plates, which extend a bone's length (physis) during the animal's early years.
The most active joints and the ones most apt to sustain injury or be attacked by disease are the synovial joints. Synovial joints, which are the horse's ball bearings, consist of two bone ends covered by articular cartilage. The cartilage within the joint is smooth and resilient, which allows for frictionless movement. Joint stability is maintained by a fibrous joint capsule, which attaches to both bones and collateral ligaments. The collateral ligaments are located on either side of most joints. They are important in maintaining stability in joints such as the fetlock, knee, elbow, hock, and stifle.
There are other ligaments within the joint itself, such as the cruciate ligaments, that also help to stabilize some joints, such as the stifle joint.
Other ligaments outside the joint cavity also lend support. A prime example involves the distal sesamoidean ligaments and suspensory ligaments that, together with the sesamoid bones, make up the suspensory apparatus and hold the fetlock in its correct position.
In addition to the fibrous joint capsule, the joint capsule itself also contains an inner lining layer called the synovial membrane. It secretes the synovial fluid that provides lubrication within the joint.
As we will see later, there are various disease processes that affect the nature of the synovial fluid and which can produce a number of soundness problems for the horse. While there is much that can go wrong within the joint, nature's overall plan for their development is masterful.
McIlwraith paid tribute to this excellent engineering job on the equine joint in one of his papers:
"The joint is a very well-engineered structure. Frictionless motion is provided by the combination of a smooth articular cartilage surface as well as lubrication of both the articular cartilage and the synovial membrane, together which make up the entire surface area of the inside of the joint.
"Shock absorption to the joint is provided by a combination of structures, including articular cartilage, subchondral bone (the bone beneath the cartilage), and the soft tissue structures (joint capsule and ligaments). Because of its resilient nature and ability to compress, articular cartilage in itself is a good shock absorber, but its thickness and overall volume is far less than bone or soft tissues. Hence, the soft tissues and the bone are the primary shock absorbers in the joint and any disease that affects bone (fractures, etc.) or soft tissue (fibrosis due to chronic inflammation) is going to interfere with this shock absorption.
"Resilience of the soft tissue is important for normal motion as well as shock absorption. It has been alluded to previously that friction comes from both articular cartilage and synovial membrane. Hyaluronic acid provides lubrication to the synovial membrane surface.
"Until recently it has been felt that it does not provide any lubrication to the articular cartilage, but more recently with some new research, it has been shown that hyaluronic acid, in addition to another protein structure called lubricin, is involved in the lubrication of articular cartilage. This substance moving over the surface of the joints is called boundary lubrication.
"A second mechanism of lubrication of the cartilage is effected by fluid being squeezed out of the cartilage onto the surface when weight-bearing occurs. When weight-bearing ceases, the fluid is absorbed back into the cartilage, ready for a next cycle of weight-bearing."
McIlwraith goes on to explain the composition of cartilage within the joint.
"On a normal microscopic section, the articular cartilage appears as a glasslike structure containing cells. The glasslike material outside the cells is called matrix. The matrix is made up of a framework of collagen, and within the framework are contained molecules called proteoglycans, as well as water. Both the collagen and proteoglycans are very important for normal function of articular cartilage."
McIlwraith then elaborates on the structure of a proteoglycan molecule. It consists of a backbone of protein with side chains of glycosaminoglycans. The glycosaminoglycans are chains of sugars with negative charges that repel each other. Because of this, McIlwraith says, the molecule is somewhat like a bristlebrush. Because of the repulsion to the side chains, and because of an attraction of water to the molecule because of the negative charges, the proteoglycans create stiffness to the cartilage and allow it to resist compression.
The proteoglycans are trapped within the collagen framework that contains them. The collagen framework also is highly important in the compressive function.
"Loss of proteoglycans or breakdown of collagen means that the articular cartilage cannot function normally," McIlwraith states.
Before leaving McIlwraith's descriptions of the joint and how it functions, it would be good to focus on his description of hyaluronic acid and the role it plays in joint functions.
"Hyaluronic acid is also known as sodium hyaluronate, or hyaluronan (the more correct term). Hyaluronic acid is a glycosaminoglycan. It is a normal component of joints, but it is generally agreed that there is some depletion of the amount in diseased joints.
"Hyaluronic acid is an integral component of both synovial fluid and articular cartilage in normal joints. Synovial fluid hyaluronic acid is produced by the synovial cells of the synovial membrane. Other hyaluronic acid that is incorporated in the matrix of articular cartilage is synthesized locally by the chondrocyte. Hyaluronic acid confers the property of viscoelasticity to synovial fluid, is responsible for boundary lubrication in the synovial membrane, and also is a factor in the lubrication of articular cartilage.
"Hyaluronic acid also influences the composition of the synovial fluid by acting as a high molecular weight barrier over the synovial membrane (called steric hindrance) and preventing active plasma components and leukocytes (white blood cells) from the joint cavity. It is also felt that solutions containing hyaluronic acid change the attraction of various other inflammatory cells.
"The hyaluronic acid that is in the articular cartilage is important in acting as a backbone for aggregations of proteoglycan molecules and aiding in the compressive stiffness of the articular cartilage."
With the above information as our base, it is time to take a look at individual joints. While it would be inappropriate to say one joint is more important than another, the fact that a horse carries more weight on its front end than the rear means there is additional stress on joints of the forelimbs.
The forelimbs of the horse bear 60-65% of the animal's weight and are subjected to greater concussive effects, especially when a horse is galloping at speed. Thus, leg injuries to racing Thoroughbreds and Quarter Horses tend to occur more frequently to the front limbs. Leg injuries to trotting and pacing Standardbreds, because of the more evenly distributed concussion, are more evenly distributed, front and rear.
While the engineering of the joints is excellent, one must wonder when looking at a drawing of the knee, for example, if the joint couldn't be a little less complex. The knee or carpal joint is composed of three main joints and numerous ligaments that keep everything tied into place. Keeping everything tied into place becomes a major concern when one considers the fact that the knee contains up to eight individual bones. (In some horses, the first carpal bone, embedded in the medial collateral ligament of the knee, might be absent.)
While they look a bit like building blocks gone awry, the seven or eight bones in the knee are arranged in two rows. Included in the top row are the radial carpal bone, intermediate carpal bone, ulnar carpal bone, and accessory carpal bone. In the bottom row are the first (when present), second, third, and fourth carpal bones.
This structure rests atop the cannon bone (third metacarpal), which is flanked on either side by splint bones (second and fourth metacarpals). Resting on top of this carpal structure is the radius.
When one considers the intricate nature of the carpal structure, the benefits of good conformation become ever more apparent. Horses which are over at the knee (buck kneed) or under at the knee (back at the knee or calf kneed) are going to be more prone to injury than are horses in which the radius blends in properly with the knee and the knee, in turn, blends in appropriately with the cannon bone.
Traveling downward, the cannon bone is connected via the fetlock joint with the long pastern bone, also known as first phalanx or P1. Also located at this junction are the medial and lateral proximal sesamoid bones, which serve as pulleys that change the direction of the deep digital flexor tendon. Although the proximal sesamoid bones are deeply embedded in and supported by ligaments, they are, nevertheless, subject to fracture.
The fetlock joint has a great degree of anti-concussive action. Adams puts it this way in his lameness book: "The stay apparatus changes the direction of concussion and weight distribution. In other words, weight is partially directed anteriorly (forward) from the distal (bottom) end of the cannon bone instead of entirely straight down. The joint is supported by the suspensory apparatus of the fetlock. The posterior cul-de-sac of the fetlock joint capsule is so constructed as to allow a great degree of motion. Of all the joints, the fetlock is subjected to the greatest stress, and at times the entire body weight may be pressed upon one fetlock joint."
Continuing our trip downward we come to another joint that connects the long pastern bone with the short pastern bone, also known as second phalanx or P2. This is the pastern joint.
The pastern joint is the least movable of the phalangeal joints. There is a minimum of anti-concussion activity at this joint.
Next, we come to the coffin joint. This joint and its function is best explained by Adams:
"The coffin joint is composed of the second and third phalanges and the navicular bone (also known as the distal sesamoid bone). This joint has a great degree of elasticity and motion because of the placement of the navicular bone and considerable anti-concussive action. Direct concussion to the coffin joint is averted by the partial distribution of weight from the second phalanx to the navicular bone. From the navicular bone, the weight is then transferred to the third phalanx (coffin bone), which descends slightly because of a yielding of the sensitive and insensitive laminae. The sole also descends slightly from pressure by the third phalanx.
"The navicular bone could not withstand great pressures but for the deep flexor tendon supporting it from behind and below. The navicular bursa has a smooth lubricating surface that reduces friction and the surface of the deep flexor tendon is closely fitted to the surface of the navicular bone.
"There is no 'pulley' action at the coffin joint because no leverage is gained; there is merely a change in direction of the weight distribution. The greatest pressure between the navicular bone and the deep flexor tendon does not occur when the foot hits the ground, but rather as the body weight passes over the foot. The central ridge of the navicular bone is subjected to greater pressure than any other portion of the bone."
That concludes our forelimb "joint" trip downward, beginning at the knee. There are, of course, two more major joints that figure into a horse's support and movement on the forehand. They are the shoulder joint and elbow joint. Because a good deal of concussion has already been absorbed by the time its effects can reach these joints, they are much less prone to injury.
The shoulder joint is devoid of ligaments, but the muscles and tendons around it hold things firmly in place and dislocation seldom occurs.
Medial and lateral ligaments stabilize the elbow joint.
Hind Limb Joints
Switching to the rear, there are two highly important joints that require discussion--the hock joint and the stifle joint.
The hock or tarsal joint is the spot where the tibia is joined with the metatarsal bones. The hock joint is a bit like the knee in that it is composed of a number of bones. There are six bones in the hock joint, but they do not have the degree of motion that is seen in the bones in the knee joint. Like the knee joint, the tarsal joint is held together by a complex set of ligaments that also enable it to function properly.
The partial flexion of the hock joint at all times aids in diminishing concussion.
The bones of the hock joint are the calcaneus and talus on the top side and the central tarsal bone, third tarsal bone, first and second tarsal bones (which are fused), and the fourth tarsal bone, all of which rest atop the metatarsal bones.
In his book on lameness, Adams described a unique aspect of the equine hock: "The oblique ridges in the tibial tarsal bone of the horse cause some differences in the action of the hock when compared with other animals in which the ridges are usually straight. As the body weight passes over the hind limb, it is not uncommon to observe a considerable outward twist of the hock joint in some horses. At the same time, the stifle joint and the toe turn in. This effect is due to the ascent of the lower end of the tibia on the oblique ridges of the tibial tarsal bone. During flexion, the oblique setting of the ridges on the tibial tarsal bone apparently aids in turning the stifle joint outward, which allows clearance past the posterior ribs."
Moving upward from the hock joint, since we have already discussed the function of the pastern, fetlock, and coffin joints when discussing the forelimbs, we come to the stifle joint, the spot at which the tibia joins the femur.
The stifle joint is the largest joint in the horse's body. One function of the stifle joint is to cause the limb to become rigid when the foot is on the ground. This is done by the contraction of muscles inside the patella.
Though this joint, similar to the knee joint in humans, is the largest in the equine body, it is not immune to injury. There are cases where there is upward fixation of the patella and the stifle and hock cannot flex, but the fetlock can. More about that next month.
Above the stifle joint is the hip joint, which is of ball and socket construction and is stabilized by strong bands of ligaments. It is the spot where the upper end of the femur fits into the socket of the hip bone. One of the ligaments that stabilizes this joint--the accessory ligament--does not occur in other domestic animals. It is found only in the horse.
The joints described above are the prime ones in the equine and the ones where problems, if they are to occur, will generally manifest themselves.
In each case, stability of the joint is facilitated by a complex network of tendons, ligaments, and muscles. When all is well in the equine world, these joints function in synchronized fashion and the horse travels along smoothly without a hitch in its gait.
As is readily apparent, however, there is much opportunity for things to go awry, especially when the joints are subjected to serious concussion, such as in racing, jumping, reining, or cutting.
Next month we will examine in detail some of the maladies and problems that can arise with the joints and their attendant stay apparatuses, as well as discuss the latest approaches in equine medicine for dealing with these problems.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals