Eric J. Northcut

“In vivo” determination of hip joint separation and the forces generated due to impact loading conditions

Numerous supporting structures assist in the retention of the femoral head within the acetabulum of the normal hip joint including the capsule, labrum, and ligament of the femoral head (LHF). During total hip arthroplasty (THA), the LHF is often disrupted or degenerative and is surgically removed. In addition, a portion of the remaining supporting structures is transected or resected to facilitate surgical exposure. The present study analyzes the effects of LHF absence and surgical dissection in THA patients. Twenty subjects (5 normal hip joints, 10 nonconstrained THA, and 5 constrained THA) were evaluated using fluoroscopy while performing active hip abduction. All THA subjects were considered clinically successful. Fluoroscopic videos of the normal hips were analyzed using digitization, while those with THA were assessed using a computerized interactive model-fitting technique. The distance between the femoral head and acetabulum was measured to determine if femoral head separation occurred. Error analysis revealed measurements to be accurate within 0.75mm. No separation was observed in normal hips or those subjects implanted with constrained THA, while all 10 (100%) with unconstrained THA demonstrated femoral head separation, averaging 3.3mm (range 1.9-5.2mm). This study has shown that separation of the prosthetic femoral head from the acetabular component can occur. The normal hip joint has surrounding capsuloligamentous structures and a ligament attaching the femoral head to the acetabulum. We hypothesize that these soft tissue supports create a passive, resistant force at the hip, preventing femoral head separation. The absence of these supporting structures after THA may allow increased hip joint forces, which may play a role in premature polyethylene wear or prosthetic loosening.

An in vivo analysis of the effectiveness of the osteoarthritic knee brace during heel-strike of gait

The objective of this study was to analyze the effects of off-loading knee braces in patients diagnosed with symptomatic unicompartmental osteoarthritis. Under fluoroscopic surveillance, 15 patients were asked to perform normal gait on a treadmill. Each patient was asked initially to walk without using a knee brace and then to walk while wearing a brace. The fluoroscopic images of the patients at heel-strike were downloaded to a workstation computer. Condylar separation angle of the knee joint and the distances from the medial and lateral femoral condyles to the tibial plateau (condylar separation) were measured. Twelve of 15 patients (80%) reported relief of pain and demonstrated condylar separation of the degenerative compartment with the use of the off-loading brace. The 3 patients who did not demonstrate condylar separation were obese, making accurate brace fitting difficult. The average change in condylar separation and condylar separation angle was 1.2 mm (range, 0.0-4.5 mm) and 2.2 degrees (range, 0.0 degrees-7.8 degrees). This study demonstrated that condylar separation of a degenerative knee compartment can be achieved with off-loading braces with subsequent subjective relief of knee pain.

A determination of ankle kinematics using fluoroscopy.

In vivo weight-bearing studies utilizing dynamic video fluoroscopy have been shown to offer an accurate and reproducible method for determining the kinematics of a joint. The purpose of this study was to evaluate translational and rotational motions of the distal tibia relative to the talus in the sagittal and frontal planes. Ten subjects, each having a normal ankle and a total ankle arthroplasty on the opposite side (Buechel-Pappas Total Ankle, Endotec, South Orange, NJ), were studied under in vivo, weight-bearing conditions using video fluoroscopy. All ten subjects were judged to have a successful arthroplasty without demonstrable pain or ligament instability. Under weight-bearing conditions, each subject performed successive motions moving from maximum dorsiflexion to plantarflexion. At maximum dorsiflexion, both the normal and implanted ankles had similar sagittal midline talar contact positions but with plantar flexion, implanted ankles had increased posterior talar contact. Contact points on the distal tibia revealed that the lateral surface contacted at the midline or posterior throughout range-of-motion with minimal translation. The medial distal tibia contacted the talus posterior on plantarflexion and often moved anteriorly with dorsiflexion. This translation described relative external rotation of the distal tibia on plantar flexion and internal rotation on dorsiflexion. The measured distances were larger for the implanted ankles with higher variability. The average range-of-motion was 37.4° for normal ankles and 32.3° for implanted ankles. This study defines the normal kinematic rotational and translational motions of the ankle joint by accurately describing the three dimensional joint orientations. The implanted ankles experienced rotational and translational motions but had contacts more posterior, possibly related to surgical technique or alterations of ligamentous tension.

Pose estimation of artificial knee implants in fluoroscopy images using a template matching technique

The paper describes an algorithm to estimate the position and orientation (pose) of artificial knee implants from X-ray fluoroscopy images using computer vision. The resulting information is used to determine the kinematics of bone motion in implanted knees. This determination can be used to support the development of improved prosthetic knee implants, which currently have a limited life span due to premature wear of the polyethylene material at the joint surface. The algorithm determines the full 6 degree of freedom translation and rotation of knee components. This is necessary for artificial knees which have shown significant rotation out of the sagittal plane, in particular internal/external rotations. By creating a library of images of components at known orientation and performing a template matching technique, the 3D pose of the femoral and tibial components are determined. The entire process, when used at certain knee angles, gives a representation of the positions in contact during normal knee motion.