A micro-architectural evaluation of osteoporotic human femoral heads to guide implant placement in proximal femoral fractures

Peer reviewedPublisher PD

What is the relevance of the tip-apex distance as a predictor of lag screw cut-out?

Using a simple mathematical formulation, the relationship between the position of the lag screw tip (relevant to both intramedullary and extramedullary devices) and the concept of tip-apex distance (TAD) was derived. TAD is widely used in operating theaters as a surgical guideline in relation to the fixation of trochanteric fractures, and in clinical studies as a predictor of lag screw cut-out. In order to visualize better this concept, the locus of points having the same TAD was plotted and the dependence of TAD on the location of the lag screw tip was also reported. It was shown that TAD should be adjusted for the size of the femoral head (a variable which varies a lot according to the sex of the patient) while no correlation was found between TAD and bone morphometry indices obtained from micro-CT data (BV/TV and Tb.Th). Therefore, these results seem to suggest that TAD lacks mechanical justification and that predictors which are based on mechanical properties, such as bone density, should be investigated further

AP view of the locus of points with a constant TAD.

<p>The locus of points (projected on an AP view) with a TAD value equal to 15, 25, 35 and 45 mm was drawn as a red surface for a spherical femoral head with a diameter of 47 mm. The blue dot depicts the position of the apex of the head. Contour lines with a constant <i>z</i> value were also plotted to visualize better the shape of the surface.</p

TAD values within a given plane.

<p>TAD values were plotted for all the possible positions of the lag screw tip within planes whose projection on plane <i>xz</i> was characterized by an angle of 45 degrees with the <i>x</i>-axis. These planes were further defined by the radial distance which separates them from the apex of a femoral head of 47 mm in diameter. The color bars provide a visual representation of the TAD values in mm.</p

C-arm for AP and lateral views.

<p>The position of the C-arm was matched to a typical view of the proximal femur with an implanted hip screw, both AP and lateral. The corresponding components of the tip-apex distance were also shown to illustrate <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071195#pone.0071195.e004" target="_blank">Eq. 1</a>. Finally, the two coordinate systems <i>xyz</i> and <i>x′y′z′</i> were also described. The axes should actually have their origin at the center of the femoral head, but were translated to make the diagrams less cluttered. <i>xyz</i> are related to an AP view while <i>x′y′z′</i> result from two rotations applied to <i>xyz</i> (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071195#s2" target="_blank">Materials and Methods</a> section). Reproduced with permission from Stryker and Baumgaertner <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071195#pone.0071195-Baumgaertner1" target="_blank">[3]</a>.</p

Relationship between micro-CT data and TAD.

<p>Tip-apex distance (TAD in mm), ratio of bone volume to total volume (BV/TV in %) as well as trabecular thickness (Tb.Th in mm) were represented for 18 cubic regions of interest (ROI) in planes 1 and 2, as depicted in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071195#pone-0071195-g001" target="_blank">Figure 1</a>.</p

Locus of points with a constant TAD for different head diameters.

<p>The locus of points with a TAD equal to the recommended threshold value of 25 mm was plotted, both as a 3D view and in an AP view, for two extremes of the range of head diameters, i.e. 35 mm and 59 mm.</p

Spherical model of the femoral head.

<p>(A) The femoral head was modeled as a sphere centered at the origin of a right-handed Cartesian coordinate system, and whose apex was depicted as a star in this projection of the head on an AP view. The femoral neck axis (at an angle of 135 degrees with the femoral shaft) was drawn as a thin black line going through the apex and the center of the sphere, while the planes of interest for the results shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071195#pone-0071195-g003" target="_blank">Figure 3</a> were represented as thick lines. (B) Cross-section through the femoral head showing the labeling of the different regions of the femoral head (ROIs) at two levels, planes 1 and 2 as shown in (A). ROIs 1–9 belong to plane 1 and ROIs 10–18 to plane 2.</p

TAD values within a given plane for different head diameters.

<p>For two femoral heads with a diameter of 35 mm and 59 mm respectively, the TAD values were plotted for a lag screw tip located anywhere within a plane defined by a radial distance from the apex equal to <i>r</i>/4 and <i>r</i> (same planes as described for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071195#pone-0071195-g004" target="_blank">Figure 4</a>).</p