Hard callus and cortex

First, the development of the whole hard callus and cortex was investigated. The variables tissue area TA and bone area fraction BA/TA were investigated for the callus or cortex in a whole (or globally) to describe the healing progression on a first and basic approach. In this animal experiment, the external fixator was medially applied, resulting in lower deformations on that side (see also Figure I.8). This influenced the two investigated variables.

Area of hard callus

The box plots in Figure I.1 show the time development of the hard callus area for the four cases (rigid lateral, rigid medial, semi-rigid lateral, semi-rigid medial). The area increases for all four cases till 6 weeks.

Figure I.1: Box plots of the hard callus area of the rigid and lateral side and both fixator types at 2, 3, 6 and 9 weeks post-op.

The three horizontal lines of each box indicate the upper quartile, the median value and the lower quartile. The solid and the dashed line show the two trends of the bone area fraction (BA/TA) for averaged values of the animals with a rigid fixation and averaged values for a semi-rigid fixation (trend estimated by eye).

Comparing the fixation types, one can clearly see two different trends for the rigid and semi-rigid fixation. The rigid fixation shows a decrease of area between week 6 and week 9 (due to remodelling) while this is not the case for the semi-rigid fixation

(Figure I.1). Without the knowledge of the other time points, one would conclude for the 6 weeks time point that both fixations lead to similar results. For the 9 weeks time point, one could even conclude that the semi-rigid fixation is desirable because more hard callus was formed. Taking all four time points into account, the hard callus area data indicates clearly a faster healing for the rigid fixation. The maximum area for the rigid fixation is assumable reached at around six weeks while the maximum for the semi-rigid is probably between six and nine weeks. This also indicates the high variability for semi-rigid fixation at 6 and 9 weeks. This example shows the necessity of investigating several time points when comparing the healing of different animal groups. However, several healing parameters should be investigated to corroborate these conclusions. For this reason, the tissue type pattern will be analysed in Chapter 5.

When comparing the osteotomy site (medial, lateral), all median values were higher for the lateral side, except for the 9 week rigid case. This confirms the histological evidence that the 9 week animals with rigid fixation show a very late stage of healing where remodelling is the dominating process. The influence of the osteotomy site on bone formation seems to be more influential than the fixation type at week 2 and 3.

The high variability between the sheep makes definite statements questionable. The lateral side is higher loaded mechanically and this could be a reason for this higher response by means of bone formation. Furthermore, the biological difference between the two sides could be the reason for this observation. It was argued, that the lateral side of the tibia is covered with more tissue providing a higher potential to heal and, therefore, this is the main reason for the faster development on the lateral side (Epari 2006).

BA/TA of hard callus and cortex

The development of the bone area fraction, BA/TA, showed two different trends: an increase of BA/TA for the callus and a decrease of BA/TA for the cortex (Figure I.2).

The median values of BA/TA four each group (rigid lateral, rigid medial, semi-rigid lateral and semi-rigid medial) could be linearly fitted (the mean increase of the four cases was 4.85% per week) and showed interesting dependencies on the fixation type and side of the bone axis (Figure I.2, top). The decrease of the bone area fraction BA/TA in the cortex could not be fitted as well (Figure I.2, bottom). The mean decrease of the four cases was 2.61% per week.

time [weeks]

slope rigid, lateral: 4.26 (r2: 0.95) slope rigid, medial: 5.80 (r2: 0.99) slope semi, lateral: 4.06 (r2: 0.93) slope semi, medial: 5.29 (r2: 0.97)

slope rigid, lateral: -2.84 (r2: 0.77) slope rigid, medial: -2.81 (r2: 0.83) slope semi, lateral: -2.51 (r2: 0.94) slope semi, medial: -2.29 (r2: 0.98)

Figure I.2: Box plots of the bone area fraction (BA/TA) of the whole hard callus for rigid and lateral side and both fixator types at 2, 3, 6 and 9 weeks post-op.

The three horizontal lines of each box indicate the upper quartile, the median value and the lower quartile. The value of r² is the coefficient of determination (for a perfect linear fit, r2 is equal to 1)..

The fixator was medially applied, resulting in lower deformations on that side (see also Figure I.8). With this information in mind, the results for BA/TA drive the impression that the mechanical stability is a key parameter for the increase of BA/TA (densification) of the hard callus. For the case of highest stability, that means the medial side on a rigid fixator, the fastest densification was found and for the case of lowest stability, that means the lateral side on a semi-rigid fixator, the slowest densification was found (Figure I.2, top). Furthermore, the osteotomy site (medial or lateral) was found to be more important for the slope than the fixation type.

Neglecting biological influences, this indicates that the mechanical influence on the

healing process depends more on the osteotomy site (due to the asymmetrical applied external fixator) than on the two different fixators.

The decrease of the cortical BA/TA agrees with the idea of functional adaptation.

During the healing process, the cortex is less loaded compared to the un-fractured situation. At the beginning of the healing process, the leg is less loaded due to pain.

After the hard callus has bridged the gap, the load is also transmitted via the hard callus and therefore reducing the strains in the “old” cortex. Despite the less precise fitting, two tendencies were found. The decrease of bone area fraction was higher for rigid fixation than for semi-rigid fixation, which is the same tendency as for the densification of the hard callus. However, higher increase of bone area fraction was found for the lateral side compared to the medial side, which is the opposite trend in the hard callus densification.

Surprisingly, both processes, the increase of BA/TA of the hard callus and the decrease of BA/TA in the cortex, were found to follow a linear trend. This is probably only valid for the investigated time period between 2 and 9 weeks. The onset of remodelling between 6 and 9 weeks will eventually lead to a stop of further densification.