Humeral (shoulder bone) fractures have become an important issue in the New Zealand dairy sector since they were first reported in 2008. The fractures affect at least 4% of dairy farms and approximately 5000 heifers are thought to be lost to the condition.
Affected farms have been observed all over the country with a number of properties having multiple animals (1-25% of replacement heifers) in the same year affected or have animals affected over multiple years.
Michaela Gibson is looking at humeral fractures for her PhD studies. She has written this article to increase awareness around the problem. Want to help out with the research? Scroll to the bottom to find out how...
The cause of fractures is not fully understood but is likely to be a combination of improving genetics for milk production, unbalanced calf/heifer nutrition and in some cases copper deficiency, preventing animals from reaching peak bone mass.
This study aimed to determine if the cannon bone or metacarpus could be used as a predictor of humeral bone structure.
In order to understand why humeral fractures are occurring, a better understanding of bone development needs to occur.
Cortical bone growth in the metacarpal (cannon bone) is limited after one year
Humeral growth is not limited after one year of age and is related to the size of the cow
Scans at the metacarpal are a good predictor of humeral structure and strength
What are humeral fractures?
The humerus is the bone up in the shoulder, as shown in the diagram below. A broken humerus is most commonly seen in 2 year old heifers from late pregnancy to about 2 months after calving, however, a small proportion of cases have been reported in 3 year old heifers.
Post-mortem examination of the broken humerus has shown that it is a complete spiral fracture due to osteoporosis (decrease in cortical bone thickness); therefore, there is no humane treatment.
The bone on the left is the cross section of a normal humerus and the one on the right is a fractured humerus. As you can see from the fractured humerus the muscle had driven up through the fracture due to the force placed in the shoulder.
Why do cows get broken shoulders?
With spontaneous humeral fractures being unique to New Zealand, there is no overseas data to provide insight into why we are seeing this condition, so case studies in New Zealand are all we have to work with. The more information collected on what is considered normal and abnormal with regards to bone development in dairy heifers, the closer we become to understanding where it all goes wrong.
Research is currently being undertaken to understand why fractures are occurring by looking at heifer bone growth and development, of which, surprisingly, little is known.
An issue that occurs when trying to identify at risk heifers, is the location of the humerus up in the shoulder is covered with muscle, making it difficult to easily access for testing.
Bones can be scanned using peripheral quantitative computed tomography (pQCT), providing quantitative data on trabecular and cortical bone properties and distribution.
This is done by scanning the bone (as seen in the photo below) and allocating bone segments to cortical or trabecular bone based on the density using a standard calibration of known densities.
The location of the humerus means that it cannot be scanned by pQCT in a live animal. This research was designed to look at whether it is possible to use pQCT scans of the metacarpus to predict bone structure of the humerus.
The metacarpus and corresponding humerus was collected from 57 animals from three different studies. As well as bone scans, size, weight, age and general breed type were also recorded.
The main traits looked at as indicated in the diagram below, were periosteal circumference (indicator of bone size), cortical thickness (bone thickness) and stress strain index (bone strength).
Periosteal circumference is the distance around the metacarpal bone. Periosteal circumference in the humerus and metacarpus increased in calves, however, growth in the metacarpus slowed in yearlings and mixed age animals. Humeral growth was not limited after 1 year old and continued to grow in the mixed-age animals, as shown by the difference in the grey vs purple lines.
Overall, there was a strong correlation between the periosteal circumference of the metacarpus and the humerus (R-squared=0.94).
Cortical thickness is the thickness of the bone (red lines in the diagram above). Cortical thickness was small in calves and increased in older animals but no difference was seen between yearlings and mixed age animals in both the humerus and metacarpus.
The figure below shows that there was a correlation between the bone thickness of the metacarpus and the humerus (R-squared=0.91).
Stress strain index
Stress strain index is a measure of a bone's resistance to breaking.
Calves had low bone strength. This would be expected from small animals that have small body size, that do not require bones to be as strong as bigger animals.
Mixed age animals had the same bone strength in the metacarpus as the yearlings but higher bone strength in the humerus. This shows that increases in bone strength are driven by increases in bone size rather than an increase in the thickness of the bone.
The figure below shows that metacarpal bone strength is indicative of humeral bone strength. The overall R-squared was 0.98.
What can we take from this?
Results from this study showed that the metacarpus is a good predictor of humeral bone structure in cattle.
Increases in bone strength in the humerus as the animal ages is likely to be a response to increases in animal size, unlike the metacarpal which does not increase in strength after a year.
This also hints at why the humerus is more likely to fracture compared to bones such as the metacarpus, due to its greater and longer growth potential.
This shows how dynamic the humerus can be in response to body growth and increases in muscular forces placed on the bone.
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With an understanding of what normal bone looks like through the ages and the relationship between the humerus and metacarpal, the same technique can be applied to bones from animals with humeral fractures. This may enable the detection of animals at risk of humeral fractures and provide an opportunity for preventative measures to be taken.
There are likely two types of fractures occurring leading to two different projects:
Fodder beet is a common winter crop in the South Island, and may be phosphorus deficient leading to poorly mineralised bones (rickets/osteomalacia) which may result in fractures, not just of the humerus, but also femur and metacarpus. For diagnosis of rickets, rib is needed along with the humerus and metacarpal.
"Traditional" heifer humeral fractures. These bones have osteoporosis, but the cause of the osteoporosis is still not clear. The current hypothesis is that it is due to intermittent periods of inadequate/unbalanced nutrition leading to poor bone deposition and inadequate development of peak bone mass + increased bone removal to supply calcium for lactation +/- Cu deficiency (about 50% of cases) which makes bones weaker. These are likely the case in animals that are not fed fodder beet and are not in the South Island.
We have a multi-pronged approach to investigating these fractures:
Collection of unfractured humerus and metacarpus (cannon bone) from the same leg in order to try and determine the cause of the fractures
A short one page survey on a selection of potential risk factors for the fractures. This survey is for both farmers that have humeral fractures in their cattle but also those that haven’t, so we can compare the two groups and see if there are obvious differences.
In spring 2020, pending funding, we aim to run a year-long prospective study with farms that regularly have fractures and those that have never had fractures to compare farm practices, soil mineral status, pasture mineral status etc.
If you are interested in updates, more information or potentially being involved in this study, contact the facebook page @masseyheiferfracture or email: K.E.Dittmer@massey.ac.nz
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Gibson M, Rogers C, Dittmer K, Hickson RE, Pettigrew EJ, Back PJ 2019. Can bone measures of the bovine metacarpus predict humeral bone structure? New Zealand Journal of Animal Science and Production 79: 8-12.