Vetstream says about puppies and their bones: https://www.vetstream.com/watkinsandtasker/html/factsheets/dog/24_265463.asp
Why is bone injury in pups serious?(back to top)
Puppies and young dogs have enormous healing potential and bone fractures, once treated, can heal rapidly and completely. When a puppy is born the ends of each bone are soft and it is from these soft parts that the bone continues to extend as the puppy grows. These areas are called the growth plates. Because the growth plates are softer they are a weak point of the bone in the growing animal. Once a dog reaches adult size the growth plates close and become bony. Any damage to the growth plates of bones prevents the bone from growing normally. If this happens to the bones in the leg this can have severe consequences and limb shortening or deformity can result.
How growth plate should look like on xray? And where are cartilage tissues (which are no radiopaque - they will not stop xray photons and are not imaged on xray photos --- "Cartilage does not absorb x-rays under normal In vivo conditions")?
There is blog post https://jsillers6760.wordpress.com/2011/02/07/june-2010-volume-six-number-1/
"Bone Growth in Canines" (Articles by Dr. James Sillers)
In the formation of long bones, there is one basic principle to remember: most bones start off as cartilage and then that cartilage is replaced by bone.
Early on, the embryonic skeleton is mainly made up of hyaline cartilage. At some point, this cartilage starts to be replaced with bone. The cartilage is considered to simply be a model for the shape the bone should take on.
The long bones grow in length by a different process. As mentioned above, most bones start off as cartilage and then that cartilage is replaced by bone. This is how the long bones grow longer. At the end of each long bone is located an epiphyseal disk that we refer to as a growth plate. This growth plate is composed of cartilage. The cartilage within the growth plate is continually dividing, sometimes faster than at other times. As it makes more cartilage, the older cartilage is replaced by bone. As more and more bone is formed, the length of the bone becomes longer. During puberty hormones stimulate the growth plate to divide at a faster rare causing rapid growth in the length of the bones. After puberty the bones stop growing because hormones cause all of the cartilage within the growth plate to become ossified (bone).
Author of the blog has several xray images for puppies. At sewen weeks growth plates are like thin lines in metaphysis:
On the left side you will see radiographs taken of a puppy seven to eight weeks of age. The arrows point to the major growth plates seen in the radiograph. Compare the radiographs taken of the puppy to the radiograph taken of the same view on an adult dog. Notice that the growth plates can no longer be seen. The age of a pup when the growth plates are closed depends on the breed of dog and which growth plate one is evaluating. The growth plates in larger breeds of dogs close at an older age than smaller breeds.
Xray of 7, 9 week pups and adult: https://jsillers6760.files.wordpress.com/2011/02/clip_image00261.jpg
The arrows are pointing to the open growth plates located in the pelvis and rear legs. Compare the radiographs token of the puppies to the radiographs of the adult dog with closed growth prates on the right.
Xray of 7week pup and adult dog: https://jsillers6760.files.wordpress.com/2011/02/clip_image0028.jpg
In earliest stages of bone development, there is no calcium here, and bone is invisible in typical X-Ray. Then there are three centers of ossification (osteogenesis), first is primary center in the middle of the bone, and then two epiphyseal centers are formed at both ends of bone. LifeMap has a picture of bone development from MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D. chapter 65. Bone Histogenesis: Secondary Ossification Centers and Joint Development
Wikipedia lists several ages for human bone development in https://en.wikipedia.org/wiki/Ossification
Third month of fetal development - Ossification in long bones beginning
Fourth month of fetal development - Most primary ossification centers have appeared in the diaphyses of bone.
Birth to 5 years - Secondary ossification centers appear in the epiphyses
At 1 month after birth human has joints made not of bone tissue, but of hyaline cartilage, which gives only light shadow on xray images: http://ww1.prweb.com/prfiles/2013/04/22/10659039/humanknee.png
Some guidelines for puppies and their soft bones by Jane Killion http://www.puppyculture.com/appropriate-exercise.html
Growth plates gradually thin as hormonal changes approaching puberty signal the growth plates to close. In puppies, this closure is normally completed by approximately 18 months old.
Until the growth plates close, they’re soft and vulnerable to injury. After sexual maturity, the growth plates calcify and the rapid cell division ends. The growth plate becomes a stable, inactive, part of the bone, now known as an epiphyseal line.
.. Injury to a growth plate can result in a misshapen or shortened limb which, in turn, can create an incorrect angle to a joint which can make the puppy more prone to yet more injuries when he grows up.
In addition to having soft growth plates at the end of long bones, a puppy’s bones in general are “softer.”
... you may find yourself clutching your puppy, afraid to let him move lest he breaks a limb. Relax! Not only is appropriate exercise not dangerous for your puppy, exercise has been shown to increase bone density in children.
Jumping off of beds and couches are major causes of spiral fractures in puppies - we are constantly on guard until our puppies reach two years old and keep them off furniture and beds unless we’re there to help them off. We also use heavy carpet pads and carpets around all furniture and beds to cushion impact, should a young (or old) dog slip by and get up on a high piece of furniture.
Probably the biggest cause of growth plate and soft tissue injury is repetitive exercise with a young puppy. So, until he’s about 18 months old, long hikes and walks are out and lots of free-play sessions are in.
Speaking of hikes, if you’re an outdoorsy type of person, you should bring your puppy along on hikes - its great socialization for puppies under 12 weeks old, and great enrichment for older puppies. But just like when you take a small child on a walk, be prepared to carry your puppy a good portion of the way. If you’re jogging or walking on a manicured trail or paved park road, consider investing a puppy stroller to put your tyke in for most of the walk.