Shchepilina O.V., Begun P.I.

Saint Petersburg Electrotechnical University «LETI»

Dynamic research of system«thighbone-bone graft-implant» of the rehabilitation period

The urgency of the rehabilitation problem during postoperative period after the hip fracture results from the fact that the thighbone traumatic injury affects the locomotor system kinematic reactions in general, thus facilitating associated disorders that do not directly result from the injury, yet worsening the patient’s life.

Despite new implant designs, improved skills of surgeons, new operation methods implemented, the results stop satisfying patients as the full recovery period reaches half a year. This is because the missing is the individual approach depending on the bone tissue condition, the fracture location. The issue of the bone graft reconstruction at the subcapital fracture location lacks attention.

However, information technologies development in medicine, particularly in trauma surgery, orthopedics and biomechanics allows achieving radically new rehabilitation technology level.

The object of the research is to develop thighbone diagnostic technique after osteosynthesis with muscle activity and elasticity module (E, MPa) taken into account at every bone graft remodeling stage. The algorithm has been developed, the calculations have been carried out and the analysis and the research have been undertaken for the “thighbone-bone graft-implant” system stress and stain behavior at various rehabilitation stages.

The following assumptions were considered while building the conceptual model:

1) thighbone bone structure is idealized to comprise two isotropic layers: cortical and spongy;

2) within the thighbone, the fissure is located at the thighbone neck cross-section and it has uniform isotropic structure, wherein its mechanical properties change at every osteotylus reconstruction stage and those are localized within the zone that is free of muscular efforts;

4) dynamic stress is applied to the thighbone center by axes X, Y, Z (www.orthoload.com).

Figure 1 represents experimental data, with the coordinate system selected (fig.1,a), axes orientation and coordinate center within the thighbone shown (fig.1,b), as well as the example of the effective load changes as a function of time (fig.1,c).

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Fig.1. Test data

As initial data, the thighbone MRT is used (fig.2,a) to build the object 3d models (fig.2,b) by means of Mimics, the computer modeling environment. With those models imported into the Solid Works software package, a solid thighbone geometric model was obtained with damages at the area of the greater trochanter (fig.2,c). The considered is the bone recovery via osteosynthesis, with two cannulated titanium screws (fig.2,d). The figure2 represents the thighbone MRT slices (a), its 3d model projection with subcapital fracture, as built via Mimics software (b), and geometric models of the thighbone neck damage (c) with the thighbone osteosynthesis (d).

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Fig.2. Benchmark data The graft bone formation process takes long time (up to 20 weeks). The

algorithm considers 4 sequential bone recovery stages (fig.3). The fig.3 contains the following designations: 1 – hematoma, 2 – cortical layer, 3 – spongy layer, 4 – cartilage, 5 – blood vessel, 6 – osteotylus, 7 – periosteum, and the represented are: the inflammation phase (fig.3a), the regenerative phase (fig.3b-c), and the remodeling one (fig.3d).

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Fig.3. Schemes inflammatory (A), regenerative (B, C) and remodeling (D) phases of callus formation at the femoral neck

At every stage, the elasticity module is given according to the diagram of the fig.4 that characterize the graft bone tissue elasticity module change during the postoperative period.

Fig.4. Change elasticity modulus callus

c

а

In terms of non-linear dynamic analysis, various rehabilitation procedures, relating to the first two rehabilitation stages, were considered. The obtained results are represented via fig.5,6. The fig.5 represents dependences of deformations appearing at the first stage with E_per=5.4kPa (a – experimental data when thigh is move aside, b – experimental data for the thigh up 30°, c – the results obtained: 1 – allowed deformation, 2 – deformation with the thigh aside, 3 – deformation for the thigh up 30°).

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Fig.5. The first stage of rehabilitation

The fig.5 represents dependences for deformations appearing at the first stage with E_per=5.4kPa (a – experimental data when thigh is move aside, b – experimental data for the thigh up 30°, c – the results obtained: 1 – allowed deformation, 2 – deformation with the thigh aside, 3 – deformation for the thigh up 30°).

The fig.6 illustrates three possible ways to walk at the second rehabilitation stage: walking with support on both legs and crutches (fig.6a), walking with support on the healthy leg (fig.6b) and walking with support on the bad leg (fig.6c) with E_per=7.6kPa (1 – support with both legs, 2 – support with the healthy leg, 3 – support with the bad leg, 4 – the deformation allowed).

Walking is an important element in the complex process of rehabilitation and positive effect on the work of many organs: cardiovascular system, on the pulmonary system improves joint mobility, prevent muscle degeneration.

The objectives of the second rehabilitation period are training in the use and development of skills crutches right away with additional support from the no-load and load on the operated leg.

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Fig.6. Walking on the second stage of rehabilitation

The following was concluded there from:

1) the tendency was discovered of the deformation depending on the elasticity module E, thus this factor must be taken into consideration when developing rehabilitation programs, particularly for the initial stages of the rehabilitation, when the bone structure has not fully restored after the damage yet, and the blood vessels are vulnerable for significant deformations;

2) putting a thigh aside during the first stage of rehabilitation, as well as walking with support on the bad leg are counter-indicative for patients with the subcapital fracture who underwent osteosynthesis.