Orthopaedic Biomechanics

Despite the generally inferior clinical performance of acetabular prostheses as compared
to the femoral implants, the causes of acetabular component loosening and the extent to
which mechanical factors play a role in the failure mechanism are not clearly understood
yet. The study was aimed at investigating the load transfer and bone remodelling around
the uncemented acetabular prosthesis.
The 3-D FE model of a natural right hemi-pelvis was developed using CT-scan data. The
same bone was implanted with two uncemented hemispherical acetabular components,
one metallic (CoCrMo alloy) and the other ceramic (Biolox delta), with 54 mm outer
diameter and 48 mm bearing diameter. The FE models of the implanted pelvis
(containing ~116000 quadratic tetrahedrals) were generated using a submodelling
approach, which were based on an overall full model of implanted pelvis (containing
~217600 quadratic tetrahedrals) acted upon by hip joint force and twenty one muscle
forces. The apparent density (ρ in g cm-3) of each cancellous bone element was calculated
using linear calibration of CT numbers of bone, from which the Young’s modulus (E in
MPa) was determined using the relationship, E = 2017.3 ρ2.46 [1].
Implant-bone interface
conditions, fully bonded and debonded with friction coefficient μ = 0.5, were simulated
using contact elements. Applied loading conditions consist of two load cases during a gait
cycle, corresponding to 13% and 52% of the walking cycle. Fixed constraints were
prescribed at the pubis and at the sacroiliac joint. The bone remodelling algorithm was
based on strain energy based site-specific formulation [2]. The FE analysis, in
combination with the bone remodelling simulation, was performed using ANSYS FE
software.
The predicted changes in peri-prosthetic bone density were similar for the metallic and
the ceramic implant. For debonded implant-bone interface, stress shielding led to ~20%
reductions in bone density at supero-anterior, infero-anterior and posterior part of the
acetabulum (Fig. 1). However, bone apposition was observed at the supero-posterior part
of the acetabulum, where implantation led to ~60% increase in bone density (Fig. 1). The
effect of bone resorption was higher for the fully bonded implant-bone interface, wherein
bone density reductions of 20 – 50% were observed in the cancellous bone underlying the
implant (Fig. 1), which is indicative of implant loosening over time. However,
implantation led to an increase in bone density around the acetabular rim for both the
interface conditions (Fig. 1). These results are well corroborated by the earlier studies
[3,4]. Implantation with a ceramic component resulted in 2 – 7% increase in bone density
at supero-posterior part of the acetabulum as compared to the metallic component, for the
debonded interface condition. Considering better wear resistant properties and absence of
metal ion release, results of this study suggest that the ceramic component might be a
viable alternative to the metallic prosthesis.

"The effects of metal ion release and wear particle debris in metal-on-metal articulation
warrants an investigation of alternative material, like ceramics, as a low-wear bearing
couple [1]. Short-stem resurfacing femoral implant, with a stem-tip located at the centre
of the femoral head, appears to provide a better physiological load transfer within the
femoral head and therefore seems to be a promising alternative to the long-stem design
[2]. The objective of this study was to investigate the effect of evolutionary bone
adaptation on load transfer and interfacial failure in cemented metallic and ceramic
resurfacing implant.
Bone geometry and material properties of 3D finite element (FE) models (intact, shortstem
metallic and ceramic resurfaced femurs of 44 mm head diameter) were derived from
the CT scan data. The FE models consisted of 170352 quadratic tetrahedral elements and
238111 nodes with frictional contact at the implant-cement (μ = 0.3) and stem-bone
interfaces (μ = 0.4) and fully bonded cement-bone interface. Normal walking and stair
climbing were considered as two different loading conditions. A time-dependant “site
specific” bone remodelling simulation was based on the strain energy density and internal
free surface area of bone [3]. The variable time-step was determined after each
remodelling iteration. The Hoffman failure criterion was used to assess cement-bone
interfacial failure.
Predicted change in bone density due to bone remodelling was very much similar in both
the metallic and ceramic resurfaced femurs (Fig. 1). Both the metallic and ceramic
implant resulted in strain reduction in the proximal regions (Region of interest, ROI 2 and
4) and subsequent bone resorption, average bone density reduction by 72% (Fig. 1).
Higher strains were generated in ROI 5 and 7, which caused bone apposition, an average
increase in bone density of 145% (Fig. 1). The tensile stresses in the resurfacing implants
increased with change in bone density; a maximum stress of 83 MPa and 63 MPa were
observed in the ceramic and the metallic implants, respectively. The tensile stress in the
cement mantle also increased with bone remodelling. Although the cement-bone interface
was secure against interface debonding in the post-operative situation, calculations of
Hoffman number indicated that risk of cement-bone interfacial failure was increased with
peri-prosthetic bone adaptation.
During the remodelling simulation, maximum tensile stress in the implant and the cement
was far below its strength. However, with bone adaptation greater volume of cement
mantle was exposed to higher stresses which, in-turn, resulted in greater risk of interfacial
failure around the periphery of the cement mantle. Both the short-stem ceramic and
metallic resurfacing component, under debonded stem-bone interface, resulted in more
physiological stress distribution across the femoral head. Based on these results, shortstem
ceramic resurfacing component appears to be a viable alternative to the metallic
design."

Short-stem resurfaced femoral implant has been found to be a promising alternative to the long-stem design,
due to reduced risk of initial femoral neck fracture and more physiological load transfer. The clinical effect
of metal ion release and continued concerns regarding the use of Metal-on-Metal bearing warrants an
investigation of an alternative material, like ceramics, as a low-wear bearing couple. The objective of this
study was to investigate the effect of a short-stem ceramic resurfacing implant, in comparison with the
metallic design, with regard to stress-strain related failure mechanisms and bone adaptation. The maximum
principal (tensile) stress in the ceramic implant, occurring at the stem-cup junction, was found to increase
from 73 MPa at the post-operative condition to 86 MPa after bone remodelling. However, the tensile
stresses generated in the cement mantle were low, around 3 MPa. The elevated bone strains occurring at
the proximal femoral neck-cup junction region were progressively reduced with bone remodelling. Bone
density distribution inside the femoral head was found to be similar to those for the metallic short-stem
design. Although bone resorption, 50 − 90% reduction in bone density was predominantly observed at the
supero-proximal femoral head, bone apposition, 10 – 30% increase in bone density occurred in the other
regions of the femoral head. Apart from better wear resistivity and chemically inert nature of ceramics, it
appears from this study that alumina ceramic is a viable alternative to the metallic design.

Biceps brachii is a muscle belonging to the flexor compartment of the arm. It has been described as one of the muscles with most frequent anatomic variations. The objective of this study was to determine the prevalence of third head of the biceps brachii muscle. A total of 14 cadavers of both sexes (13 men and 1 women) with different age group were used for the study in the Morphology Laboratory at the University of Pamplona. On the right side the origin of extra head biceps brachii muscle was given bicipital groove level in relation to the distal portion of the pectoralis major muscle, the distal insertion was joining with the short head of the biceps brachii. In the left arm of the second body, extra head biceps brachii muscle originated from the anterolateral surface of the humerus between coracobrachialis muscle insertion and origin of brachialis muscle; its distal insertion occurred in the long head of biceps brachii muscle. The dual origin of the third head of the biceps might contribute to supination of forearm, as the muscle origin appeared in a lateral position relative to the rotational axis of the arm. The knowledge of such anatomical variations is important for anatomists in the academic study and clinicians in order to avoid errors in the diagnosis or undesirable consequences during surgery, procedures safer and more efficient rehabilitation.