| Author | Post |
|---|
4Vanity
Member
| Joined: | Mon Apr 16th, 2012 |
| Location: | |
| Posts: | 2 |
| Status: |
Offline
|
|
Posted: Mon Apr 16th, 2012 08:38 pm |
|
I have just read your 2008 article. It is very much what I have always believed as far as the full growth span of a horse and furthers my own personal feeling that horses should be given more time to fill in prior to riding/training.
I write because of a friend wherein one of her Lusitanos, a 4.5 year old now gelded, was xrayed for prepurchase. The horse cleared all vetting, except for one foot, wherein it was noted that the two navicular bones had not fused. I believe 2-3 cm gap about 1" long. The horse did not show signs of lameness and was working under saddle quite well. However, the vet indicated it was a flaw so to speak and that navicular disease, while not apparent, could not be discounted in the future. I have read also that in some instances horses can have issues wherein the blood flow is limited to an extremity and thereby causing slower growth. Additionally, I have personally considered for my own horse stem cell therapy and wonder if this course of action has been experienced by anyone. I am told it helps build cartilege, so I wondered "why not bone". Also, has anyone ever had experience with a situation such as this and was there any special outcome of note. Appreciate anyone's input on this. I have also spoken to other breeders who have not experienced this with their lusitanos. Thanks!
|
DrDeb
Super Moderator
| Joined: | Fri Mar 30th, 2007 |
| Location: | |
| Posts: | 2419 |
| Status: |
Offline
|
|
Posted: Mon Apr 16th, 2012 10:44 pm |
|
Vanity, you are a little confused. There is but a single navicular "bone" in each of the horse's limbs, and there is no fusion process that this "bone" undergoes. I refer to the navicular "bone" in quotation marks because technically speaking, the navicular is not a bone but an ossified intratendonal bursa, otherwise known as a sesamoid. There are three sesamoid bones in each of a horse's forelimbs: the two that support the ankle joint, plus the navicular sesamoid which is interposed at the junction of the short pastern and coffin bone. In each hind limb, there are four sesamoid bones: the three that pertain to the forelimbs, plus each hind limb has a patella.
Sesamoid bones all have their origin as ossified intratendonal bursae, and as such, they do not undergo the same kind or process of growth as true bones. Unlike true bones, sesamoids lack a periosteum and they also do not have epiphyses or cartilagenous growth plates. They enlarge by a process of accretion at the edges, which accretion is mediated by osteoblast/osteoclast cells which are mobile in the surrounding tissue.
I think what you are trying to say -- or what the vet was trying to say and you misunderstood -- is one of two things. Either the animal was showing lesions of the navicular sesamoid -- a common reason why horses flunk prepurchase exams. Or else the animal still had "open" growth plates in the distal limb, either at the top of the coffin bone or at either end of the short pastern. This would be quite rare but possible.
Neither condition would argue particularly either for or against starting horses under saddle at age 2. The correct arguments against this practice are presented in the "Ranger piece", i.e. the skeletal maturation paper to which you refer, which is posted in our main website at http://www.equinestudies.org under "Knowledge Base". It is of some concern to me that people who read this paper do so carefully so as to not make it into a polemic against starting 2 year olds. I present the information so that the consumer or horse owner may decide for herself what may be best to do. -- Dr. Deb
|
4Vanity
Member
| Joined: | Mon Apr 16th, 2012 |
| Location: | |
| Posts: | 2 |
| Status: |
Offline
|
|
Posted: Tue Apr 17th, 2012 12:20 am |
|
Thank you Dr. Deb, I was 3rd party to the information, but I will pass along this discussion.
P.S. As far as lesions go, their definition seems quite broad, but can I safely believe that any type of lesion on the navicular sesamoid would be a negative concern for any horse?
Thanks in advance.
|
David Genadek
Member
|
Posted: Tue Apr 17th, 2012 11:23 pm |
|
Deb,
Is a sesamoid less likely to break or shatter than a bone?Last edited on Tue Apr 17th, 2012 11:23 pm by David Genadek
|
DrDeb
Super Moderator
| Joined: | Fri Mar 30th, 2007 |
| Location: | |
| Posts: | 2419 |
| Status: |
Offline
|
|
Posted: Wed Apr 18th, 2012 12:36 am |
|
No; about equally likely. The substance of the bone and the substance of the ossified intratendonal bursa are similar; both are 'molded' upon a pre-existing template consisting of a three-dimensional mesh of collagen fibers, into which calci-apatite is then deposited.
In the horse, broken (ankle) sesamoids occur fairly often simply because they're in a position to strike a rock. They also get torn in half sometimes in the process of the horse "pulling" a suspensory.
Fractures of the navicular bone don't usually occur at all, except when the bone has already been riddled with "lollipop" lesions, due to navicular disease. It may then fall apart almost with no provocation.
Fractures of the patella are extremely rare in horses, occurring essentially only due to the animal having been kicked directly on the stifle. -- Dr. Deb
|
David Genadek
Member
|
Posted: Wed Apr 18th, 2012 12:45 am |
|
| Why sesamoid and not bone?
|
DrDeb
Super Moderator
| Joined: | Fri Mar 30th, 2007 |
| Location: | |
| Posts: | 2419 |
| Status: |
Offline
|
|
Posted: Wed Apr 18th, 2012 05:44 am |
|
OK, we go over this in anatomy class, so here's the reason again:
Skeletal bones, those found behind or below the head, are formed through an embryological process which involves successive stages of replacement. All bones (and sesamoids) begin as 'wads' of collagen fibers that have the general shape of the bone or sesamoid they are intended to become. And each is, of course, located in that part of the embryo where it properly belongs.
Sesamoids begin by being located either within, or immediately adjacent to, a tendon. They occur at the points in the body where they do because the tendon involved has to go around some kind of fairly sharp curve. The function of a sesamoid is to strengthen the tendon at the point of highest pressure, at the apex of the curve; and also to serve as a 'spacer' that moves the tendon's point of pull away from the center of rotation of the joint.
"Real" bones are, of course, the elements that actually form the joints.
Both sesamoids and real bones undergo a first conversion of matrix substance, in which the collagen 'wad' becomes completely gummed up with cartilage. This is also true of bursae that do not normally become ossified. However, at this stage bones also acquire the thin, tough membrane that surrounds them which is called the periosteum. Neither bursae nor sesamoids form a periosteum.
While bursae that are going to remain as bursae retain a fibro-cartilagenous constitution, sesamoids and real bones both undergo a second conversion of matrix substance, in which most of the cartilage is removed (by chondroclast cells) and replaced by calci-apatite (by osteoblast cells).
Here is where we find a second set of differences -- in real bones, the process of the osteoblasts laying down calci-apatite is a highly complex one, such that the osteoblasts organize themselves into 'gangs' which create the incredibly complex Haversian system by which they simultaneously entomb themselves in the calci-apatite that they secrete, but also provide for the complete interpenetration of blood vessels, so that each osteoblast in its lacuna ("hermit's cell") may continue to receive the life-giving oxygen and glucose from the capillary blood supply. Sesamoids have no such interior organization, and this, and the fact that the periosteum which they lack also carries an abundant blood supply and houses many osteoblasts, is why fractured sesamoids tend to heal rather badly if at all.
A third difference that you may recall is that "real" skeletal bones replace the original cartilage in such a manner that a thin strip of embryonic cartilage ("growth cartilage") is preserved at either end of the bone shaft, so that the growth cartilages separate the shaft of the bone from the bone-ends or epiphyses. These remnant cartilage segments are the "growth plates"; no sesamoid has any growth plate, because sesamoid cartilage is not replaced in the same organized way as that of the long bones.
Now this discussion has been limited to "bones behind and below the skull", which because those bones are pre-formed in cartilage but then re-matrixed in calci-apatite, are called "replacement bones". In the skull, there is a mixture of replacement bones and what are called "dermal bones". The dermal bones form the top of the braincase and some other exterior parts of the skull. They start out as plates made of collagen, like all the other firm structures; but they never become cartilagenous. Rather, the dermal bones "precipitate" calci-apatite by activity of osteoblasts from the epidermis in which these bones are embedded. They grow by accretion at the edges. The difference between dermal bones and sesamoids is that the dermal bones are not located within or near tendons; and also that they do possess a periosteum.
Hope this explanation helps. It is important to know that sesamoids are not really bones, because in the horse the navicular and ankle sesamoids are located in places subject to a good deal of mechanical stress and are frequently involved in injury and lameness -- but because they're sesamoids, lacking a periosteum and good blood supply, they don't heal real well. -- Dr. Deb
Last edited on Wed Apr 18th, 2012 05:46 am by DrDeb
|
ruth
Member
| Joined: | Sat Oct 20th, 2007 |
| Location: | |
| Posts: | 51 |
| Status: |
Offline
|
|
Posted: Wed Apr 18th, 2012 08:06 am |
|
| Dr Deb, could the osteoblast process in the horse be compromised by osteoporosis, or is this a disease of omnivores such as humans, and not herbivores? Regards, Ruth
|
David Genadek
Member
|
Posted: Wed Apr 18th, 2012 01:44 pm |
|
| Thank you!!! My brain can only take in so much and the class covers soooo much. I will start grappling to understand one concept by the time I have it digested it your 20 concepts beyond. That just means I need to do the class more times There is no way anyone could get it in one go.
|
Current time is 10:56 pm | |
|
|
|