Lameness Imaging in the Sports Medicine Era: It’s Not Fractures Anymore

1. Introduction

The use of imaging modalities such as radiographs, ultrasound, nuclear medicine, computed tomography, and magnetic resonance imaging (MRI) have all aided in creating methods to identify abnormalities in the equine patient. Ten years ago, equine patients would have a lameness that was easily identified and imaging yielded a diagnosis. However, as owners and trainers have become more in tune with the equine athlete and the imaging modalities have become more advanced, it is not uncommon to have 6 to 10 imaging abnormalities identified without a clear or single diagnosis as the cause for the lameness. This shift from surgical lameness to sports medicine with the prevalent diagnosis being soft tissue injuries has led to the creation of the American College of Veterinary Sport Medicine and Rehabilitation.

With all of the existing imaging modalities and the increased availability of standing MRI and nuclear medicine to the practitioner, it is rarely the availability of the modality that is the limitation. In certain cases, an accurate or specific diagnosis is still lacking. In the equine realm, most imaging interpretation is done by practitioners or specialists with experience; however, board-certified radiologists have an integrated knowledge of all of the modalities and how they work together to help determine the specific modalities that have the best chance of seeing lesions and can help make a diagnosis. This knowledge allows for understanding the limitations of the modality and furthermore, understanding that a negative study provides both a list of differential diagnoses and excludes many differential diagnoses, which is something that is sometimes overlooked. 

The purpose of this review is to discuss the use of radiographs, nuclear medicine, and standing MRI in the equine patient and describe the limitations and methods to overcome these limitations.

2. Radiographs of the Neck: How Good are We at Injections? 

Radiographs of the cervical spine are now available to practitioners, thanks to digital radiographic systems. With digital radiography and a small X-ray generator, which used to only be available with larger X-ray tubes, images of the caudal cervical spine to T1 have become part of standard practice. This has led to increased diagnosis and treatment of neck pain in horses. However, have you ever wondered how skilled we are placing a needle in the joint and keeping the material in the joint recess?

At Michigan State University, a resident injected the cervical facet joints of 10 horse from C2–3 to C6–7 using contrast medium and 2 different techniques. The results are what you would expect. The C2–3, C3– 4, C4 –5, and even C5– 6 were repeatably injected with a majority of contrast material present inside the joint. Injection of C6–7 was much more variable. When the facet joints were missed at C6–7, the contrast material was injected in C7-T1 or just less than 50% of the contrast medium within the C6-C7 joint.

The technique to inject cervical facets with ultrasound guidance will be reviewed,^1,2 but briefly, a curved 8 MHz microconvex or linear >7 MHz ultrasound probe should be used. A 20-gauge needle can be used to inject the joint; however, if hyaluronic acid is being administered, an 18-gauge needle is recommended. The probe should be oriented in a dorsoventral direction and the joint identified as a V-shaped hypoechoic region and the joint capsule can be seen. The needle should be directed into the V-shaped joint space. When injecting, minimal fluid should be seen around the bone and fluid placed in the joint will cause expansion of the joint capsule.

3. Nuclear Medicine: The Underused and Over-Interpreted Modality

Nuclear medicine (also called scintigraphy) is performed using a radiopharmaceutical that binds specifically to hydroxyapapite crystals.³ Basically, rather than a radiograph that sends radiation through a bone to a detector to see how much can pass through, nuclear medicine provides a radioactive source to the horse and then, with the help of a binding agent, finds areas where osteoblasts are turning over bone. The difficulty with this modality is that bone turnover occurs for numerous reasons. The physis of a foal will normally have a high degree of osteoblast activity and even fractures can continue to heal for up to 3 years after the initial insult. Therefore, the main limitation of nuclear medicine is that it shows bone remodeling, not a source of inflammation or pain. Increased activity can help determine a region of interest for follow-up with radiographs. However, even osteophytes on radiographs do not necessarily correlate to a lameness. The main goal of the nuclear medicine study is to take a complicated multilimb lameness and see if it can be narrowed down to specific regions that cannot be characterized during the physical examination.

Soft tissue can be evaluated by nuclear medicine. This is done based on the premise that increased blood flow will be present when an inflammatory process is present and, therefore, performing the imaging scan at 5 min after injection will show the “soft tissue phase” of imaging. This can also be done using technetium pertechenate without a binding agent to just see the circulation of the radiopharmaceutical. Imaging immediately after injection can be done to assess blood flow (called the “vascular phase”); if a vascular anomaly is suspected, such as blood clot or vasculitis, unbound technetium can be used to look at the perfusion of the distal limbs. Case examples of this will be provided in the oral presentation.

4. Standing Magnetic Resonance Imaging and Elastography: A Fad or Here to Stay?

There is probably no more widespread expansion of imaging in the last 10 to 15 years that is greater than the use of MRI on the equine athlete. From soft tissue injuries to cartilage damage, MRI has taken the equine market by storm. Also, with the availability of standing MRI, general practitioners and specialists have developed equine imaging facilities to focus on the equine athlete and subtle lameness that is the most common presentation being seen by the equine practitioners.⁴

In addition, a new modality called elastography using ultrasound has recently been reported and shown to be repeatable and easy to use on normal horses to assess tendons and ligaments in horses.⁵ A paper from North Carolina State University showed a difference in the elastography or “stiffness” of tendons in weight bearing as well non-weight bearing positions. This program is now available with portable ultrasound units and may become more widespread. More research needs to be done; however, there is a chance this may serve as a method to identify microtears prior to core lesions in horses.

Standing MRI is by far more widespread; however, the limitation of this system is the field strength and patient motion. Since standing units have a small magnetic field, allowing them to be put in any facility and requiring minimal maintenance and power, the resolution is considerably less when compared to high-field systems, especially when smaller structures are being imaged, such as cartilage that is less than 5 mm thick. The level of detail that is discussed by clinicians versus what can be seen in a research environment is quite different. Standing MRI can be used to evaluate tendons and ligaments, bone edema, and even fractures.⁶ The size and severity of the lesion must be enough to allow detection using a low-field system. Limitations with magic angle artifacts and simple image noise makes over-interpretation easy, especially when a subtle lameness is present. Also, it is not uncommon to find 8 to 10 subtle lesions in a foot on standing MRI and the question is “which one is causing the problem?”

Ultimately, this one-hour lecture will show several case examples of state-of-the-art imaging available to practitioners and specialists alike. A major limitation in imaging of the equine patient is that currently there seems to be a lack of understanding of some of these modalities and, therefore, they are not used to their full potential. [...]