Fractures are a major cause of morbidity, death, and health-care expense in both humans and horses.1,2 Catastrophic fractures in horses, which result in euthanasia without treatment, also create a financial loss for horse owners.2 There is strong evidence that accumulated bone tissue modeling and remodeling in response to exercise can result in reduced bone quality in racehorses and contribute to catastrophic fractures.2–5 Bone quality describes the characteristics of a bone, other than bone mass, that contribute to its ability to resist fracture.6,7 Improved prevention strategies are needed to reduce the incidence and impact of fractures. To be successful, a fracture prevention strategy must be able to identify subjects that are at increased risk of fracture. To achieve this, assessment of an animal's bone quality or bone tissue resistance to fracture is needed. The assessment should be easily applied in a clinical setting in a straightforward and minimally invasive manner. In addition, practical, easily applied methods for monitoring the fracture resistance of bone tissue and response of bone tissue to an intervention would be a valuable tool for clinical investigators.
Currently, the most widely studied methods used to identify humans at risk of fracture are based on measurements of bone density.8 Although bone density is one of the determinants of fracture risk, other factors such as bone turnover rate, microdamage accumulation, bone matrix properties, and bone geometry also contribute to bone quality.6,7 Therefore, methods for in vivo assessment of bone quality beyond traditional bone density–based modalities are needed to identify individuals at risk of fracture.
A novel, clinically applicable in vivo bone indentation technique known as RPI has been used to assess bone tissue mechanical properties in people.9–11 This technique can be used to distinguish between a group of patients with osteoporosis-related fractures and an age-comparable control group.9 The RPI instrument used in those studies9–11 consisted of a reference probe (22-gauge hypodermic needle) positioned on the surface of a bone and a test probe (coaxial within the reference probe) that indented the bone surface. The instrument could perform a series of up to 20 indentation cycles and record measurements of total InDist and InDist increase from the 1st to 20th cycles. The distances were measured with respect to the reference probe. This testing protocol required that a patient remain still, and 5 or more separate measurements were obtained through the skin over a period of 8 to 10 minutes. Total InDist and InDist increase are related to the ability of bone to resist microfracture.9,12
To be adopted for use in horses, a clinically applicable method of assessing the fracture resistance of bone would ideally be performed on standing animals because anesthesia is costly and anesthetic recovery adds risks of morbidity and death. However, performing multiple 20-cycle indentation measurements with the previously described RPI instrument without limb movement in a standing horse would be difficult. Therefore, an RPI instrument that involved measurement of a single impact indentation cycle into a bone surface was developed.13,14 This instrument is portable and has a handpiece that contains a single, solid indentation probe that penetrates the skin and is subsequently impacted into a bone. The handpiece is connected to a laptop computer by a USB cable. This HRPII potentially may have wide applicability as a clinical test of fracture resistance of bone tissue in horses, other domestic animals, and humans.
The previously described RPI instrument was designed for use through the skin of people.9,10,12 Because of the coaxial design of the probes, it was unnecessary to measure the effect of the skin on in vivo measurements for human patients. In contrast, the HRPII uses a single indentation probe, and the effect of testing through the skin is unknown. Furthermore, the relationship between in vivo and ex vivo bone indentation measurements in the same horse by use of an RPI instrument (such as the HRPII) is unknown. This information is important for the interpretation of longitudinal in vivo investigations and relating those findings to postmortem evaluations, when applicable.15
The objectives of the study reported here were to describe the use of an HRPII in horses without bone fractures; determine BMSi values for 6 regions of third metacarpal bones; characterize the influence of age, sex, and body weight on BMSi measurements; determine the influence of skin on BMSi measurements; and compare in vivo and ex vivo BMSi measurements. Results of this study will help investigators assess the RPI technique in horses and the influence of various testing conditions on BMSi values.
Supported in part by National Science Foundation grants CMMI-1031056 (Lescun and Chandresekar) and CMMI-1031244 (Hansma and Yang). Development and construction of the handheld reference point indentation instrument used for this work was supported by the National Institutes of Health (grant No. RO1 GM 065354).
Dr. Hansma is an employee and member of the board of directors of Active Life Scientific Inc, which produces reference point indentation instruments.
All data collection for this study was performed at Purdue University.
Bone material strength index
Coefficient of variation
Handheld reference point indentation instrument
Reference point indentation
Osteoprobe RPI instrument, Active Life Scientific Inc, Santa Barbara, Calif.
BioDent RPI instrument, Active Life Scientific Inc, Santa Barbara, Calif.
SAS, version 9.2, SAS Institute Inc, Cary, NC.
Microsoft Excel, version 14.0.7128.5000, Microsoft Corp, Redmond, Wash.
Volkering ME. Variation of skin thickness over the equine body and the correlation between skin fold measurement and actual skin thickness. Doctoral thesis, Faculty of Veterinary Medicine, University of Utrecht, Utrecht, The Netherlands, 2009.
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