1st Joint ESMAC-GCMAS Meeting - Análise de Marcha

O-37
SUBJECT SPECIFIC HIP GEOMETRY AFTER THP INFLUENCES
HIP JOINT REACTION FORCES DURING GAIT
Lenaerts G, Dra 1-2 , Jonkers I, Phd 1 , Van der Perre G, MS, Prof 2 , Spaepen A, MS, Prof 1
1 Dept. of Biomedical Kinesiologym FaBeR, Katholieke Universiteit Leuven, Belgium
2 Dept. of Mechanical Engineering, FTW, Katholieke Universiteit Leuven, Belgium
Summary/conclusions
Subject specific hip geometry (especially NSA) after total hip joint replacement alters the joint
reaction forces at the hip and affects implant loading.
Introduction
Each year, 17 000 hip prostheses are implanted in Belgium. 5 – 10% need revision because of
implant loosening. Several factors have been identified to predispose implant loosening.
Musculoskeletal loading is often reported as an important factor affecting the biological
processes involved in bone remodeling and primary fixation of implants. In patients with THP,
the subject specific anatomy of the hip implant determines the moment generating capacity of
the surrounding muscles. Our previous work [3] reported a decrease in moment generating
capacity of the hip abductors when a subject specific model was used that incorporates RX
based values for femoral neck length (NL), femoral neck-shaft angle (NSA) and width of the
pelvis (WP). As a result, we reported marked changes in calculated muscle (co-) activations
using a static optimization approach. The present work evaluates to what extent hip contact
forces during gait are influenced by subject-specific geometry of the hip and the resulting
changes in muscle activation balance during gait.
Statement of clinical significance
A better insight in the biomechanical factors influencing hip loading during gait after THP will
contribute to an enhanced understanding of the factors affecting initial implant fixation and
eventually prevent implant loosening.
Methods
NL, NSA and WP were measured in 20 subjects, based on digitized post-operative RX-images
(Imagica, GreyStone Inc). A deformable musculoskeletal model of the lower limb was
adjusted to incorporate NL, NSA and WP for all patients (SIMM, Musculographics). NL
varied from 41 mm to 86 mm, NSA between113° and 144°, PW between 315 mm and 402
mm. Kinematic and kinetic data obtained from a normal gait trial were imposed to each
individualised model to calculate (1) joint moments (2) individual muscle force generating
capacity and (3) muscle moment arms over the gait cycle. Muscle activation patterns balancing
the external joint moments were computed using a static optimisation algorithm, minimizing
the sum of the muscle forces (Matlab, MathWorks Inc.). The 3D hip reaction forces were
computed taking into account the muscle forces resulting from the muscle activation patterns,
as well as external forces (the ground reaction forces – inertial forces and gravity). This
analysis was repeated for a model with halve hip abductor force generating capacity,
mimicking hip abductor weakness after surgery. The resulting changes in muscle activations
were imposed[1]. Peak reaction forces during single limb stance as well as the associated
inclination of the reaction force in the sagittal and the frontal plane are reported.
Results
The peak reation forces in the undeformed model are within the range reported in literature [2].
The changes in muscle activations due to the modified geometry introduced changes in peak
reaction forces (Figure 1). Consistent changes were most pronounced for the mediolateral
component showing a decrease in the peak mediolateral component with increasing NSA.
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