Developing a quantitative measurement system for assessing heterotopic ossification and monitoring the bioelectric metrics from electrically induced osseointegration in the residual limb of service members

Abstract

Poor prosthetic fit is often the result of heterotopic ossification (HO), a frequent problem following blast injuries for returning service members. Osseointegration technology offers an advantage for individuals with significant HO and poor socket tolerance by using direct skeletal attachment of a prosthesis to the distal residual limb, but remains limited due to prolonged post-operative rehabilitation regimens. Therefore, electrical stimulation has been proposed as a catalyst for expediting skeletal attachment and the bioelectric effects of HO were evaluated using finite element analysis in 11 servicemen with transfemoral amputations. Retrospective computed tomography (CT) scans provided accurate reconstructions, and volume conductor models demonstrated the variability in residual limb anatomy and necessity for patient-specific modeling to characterize electrical field variance if patients were to undergo a theoretical osseointegration of a prosthesis. In this investigation, the volume of HO was statistically significant when selecting the optimal potential difference for enhanced skeletal fixation, since higher HO volumes required increased voltages at the periprosthetic bone (p = 0.024, r = 0.670). Results from Spearman's rho correlations also indicated that the age of the subject and volume of HO were statistically significant and inversely proportional, in which younger service members had a higher frequency of HO (p = 0.041, r = -0.622). This study demonstrates that the volume of HO and age may affect the voltage threshold necessary to improve current osseointegration procedures.

FIGURE 1

A unilateral hierarchical model was assembled as a representative image consisting of skin (purple) adipose tissue (yellow), musculature (pink), bone (blue), bone marrow (orange), and internal organs (green) (a). Each tissue type was assigned a specific conductivity using SCIRun. A large serpentine-like mass of HO was identified in the medial aspect of the residual limb, and was demonstrated in more detail in an axial cross section of the affected limb (b).

FIGURE 2

Three dimensional reconstruction of a service member with no visible signs of HO.

FIGURE 3

Three dimensional reconstruction of a service member with 26.53 cm³ of HO.

FIGURE 4

Three dimensional reconstruction of a service member with 74.25 cm³ of HO.

FIGURE 5

Three dimensional reconstruction of a service member with 115.96 cm³ of HO.

FIGURE 6

External electrode placement was standardized by placing 2 electrodes bands 2 cm from the most distal and proximal ends of the osseointegrated implant to create a homogenous electric field at the bone–implant construct.

FIGURE 7

Electric field (a) and current density (b) distributions for service member 2 using a potential difference of 2 volts.

FIGURE 8

Current densities in the distal residual limb for the 2 volt potential difference are shown for each patient in the study. The critical threshold for current density (1.8 mA/cm²) is indicated by the horizontal dashed line.

FIGURE 9

General trend lines for each service member selected in the study demonstrate that optimal potential difference may be selected using an algorithm on a patient-specific basis.