Low-intensity pulsed ultrasound enhances callus consolidation in distraction osteogenesis of the tibia by the technique of lengthening over the nail procedure

Distraction osteogenesis (DO) using an external fixator has been successfully used in limb lengthening for short stature or limb length discrepancy. The consolidation of regenerated callus during DO can be affected by various factors including host factors (age and underlying disease), local factors (adjacent soft tissue condition and concomitant infection), and surgical factors (surgical technique, stability of the frame, and modulation of the distraction rhythm) [1–4]. Long-term external fixation, related to delayed consolidation, may lead to complications such as pin site infection and adjacent joint contracture, as well as subsequent socioeconomic and psychological burdens on the patient [3, 5].

To reduce these complications and burdens on the patient, several techniques have been employed to enhance callus maturation. Local application of autogenous and allogenous iliac bone, biphasic calcium phosphate [6], platelet-rich plasma (PRP) [7, 8], and demineralized bone matrix (DBM) [9] have been used to induce biological stimulation of callus consolidation. Furthermore, systemic application of parathyroid hormone [10], growth hormone [11], and bisphosphonate [12] have been investigated. However, further investigation of the optimal dose and potential risk of complications of these biological materials is required for their wide clinical use.

Low-intensity pulsed ultrasound (LIPUS), which has been widely accepted for promoting the fracture healing process [13–17], may be another option for adjuvant therapy to accelerate consolidation of the regenerated callus. Although there have been several in vivo studies evaluating the efficacy of LIPUS on callus maturation during DO [18–23], there have been limited clinical trials focused on the efficacy of LIPUS during DO [24–26]. Therefore, this clinical investigation was performed to evaluate the efficacy of LIPUS during DO under the hypothesis that LIPUS would enhance callus maturation.

The medical and radiographic records of patients who underwent tibial lengthening in our institute from October 2009 to October 2015 were reviewed. Only patients who were skeletally mature, underwent simultaneous bilateral tibial lengthening over the nail, and had no medical illness (such as bone metabolic disorders or neuromuscular disorders) or history of trauma were included. Patients who developed an infection (including pin-site infection and acute osteomyelitis) during DO, received adjuvant therapy other than LIPUS, exhibited a lack of compliance, or had incomplete medical and radiographic records were excluded. A total of 30 patients (60 tibiae) were enrolled in this study. The mean age at tibial lengthening was 22.1 years (17.5–34.0), and the patients were followed for an average of 5.6 years (1.7–8.2).

Fifteen patients (30 tibial segments) were classified as the LIPUS group (Fig. 1), and another 15 patients (30 tibial segments) were classified as the control group (Fig. 2). The demographic data of the LIPUS and control groups are presented in Table 1. No significant differences between the groups were found for the assessed demographic data including sex, age at operation, preoperative height, BMI, and smoking history. In addition, no significant differences were found between the groups in terms of the initial tibial length and amount and percentage of tibial lengthening (Table 2).

Variable	LIPUS group	Control group	P-value
Number of patients	15	15
Number of tibial segments	30	30
Sex (M:F) (number of tibia)	24:6	25:5	1.000†
Age at operation (years)*	22.1 (17.5 to 34.0)	20.6 (17.9 to 25.4)	0.155‡
Preoperative height (cm)*	165.4 (153 to 174)	163.2 (147 to 173)	0.073‡
BMI (kg/m2)*	21.7 (17.2 to 26.2)	21.8 (17.4 to 27.8)	0.941‡
Smoking history (smoker: nonsmoker) (number of tibia)	2:28	4:26	0.671†
Follow-up (years)*	5.3 (1.7 to 8.2)	5.8 (3.9 to 7.8)	0.790‡

LIPUS, Low-intensity pulsed ultrasound

*Values are expressed as mean, with range in parentheses

† Fisher’s exact test, ‡ Mann-Whitney test

Classification	LIPUS group (N = 30)	Control group (N = 30)	P-value†
Initial tibial length (mm)*	318 (262 to 353)	313 (258 to 360)	0.383
Amount of lengthening (mm)*	82 (70 to 105)	76 (50 to 100)	0.065
Lengthening percentage (%)*	25.8 (20.3 to 36.6)	24.7 (13.9 to 35.5)	0.350
External fixator index (days/cm)*	29.4 (16.5 to 44.8)	29.6 (17.3 to 45.8)	0.579
Healing index (days/cm)*
Anterior cortex	36.6 (21.7 to 51.6)	57.5 (32.5 to 64.8)	<  0.001
Posterior cortex	24.3 (15.3 to 43.0)	25.9 (16.9 to 44.8)	0.367
Medial cortex	32.5 (19.5 to 47.6)	44.2 (29.3 to 53.4)	0.002
Lateral cortex	29.4 (16.5 to 44.8)	29.6 (17.3 to 45.8)	0.579

LIPUS, Low-intensity pulsed ultrasound

* Values are expressed as mean, with range in parentheses

† Independent t-test

There was no significant difference in the EFI between the groups. However, significant differences were found in the HIs of the anterior and medial cortices (p <  0.001 and p = 0.002, respectively) (Table 2). The HIs of those cortices in the LIPUS group (mean of 36.6 days/cm and 32.5 days/cm, respectively) reflected their significantly faster healing compared to the control group (mean HI of 57.5 days/cm and 44.2 days/cm, respectively).

We performed a clinical investigation on the efficacy of LIPUS during DO. The findings of the current study showed that LIPUS stimulated a more cylindrical, more homogenous, and denser type of callus formation in the early stage of DO (at the end of distraction phase) and promoted faster callus consolidation, especially in the anterior and medial cortices.

LIPUS is a mechanical stimulation method using high-frequency acoustic pressure waves. It is speculated that LIPUS exerts low-magnitude mechanical stress at the application site and acts through a functional loading effect [30]. This may provoke a cascade of biological responses during cellular reaction processes in the inflammation, angiogenesis, chondrogenesis, and endochondral and intramembranous ossification stages, as well as the bone remodeling period [18, 30]. Accordingly, LIPUS stimulates differentiation and proliferation of osteogenic cells, activation of osteoblasts, and synthesis of osteocalcin [23]. Expression of messenger ribonucleic acid (mRNA) and synthesis of fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and prostaglandin E2 are also increased via LIPUS stimulation [17, 31–34].

To produce acceptable outcomes with LIPUS therapy, a well-formulated LIPUS protocol is required. The LIPUS protocol established in previous studies is a frequency of 1.5 MHz, a signal repetition rate of 1 kHz, a pulse width of 200 μs, and an intensity of 30 mW/cm2 with a 20-min application time per day [23, 26, 28, 35]. Because these previous studies achieved satisfactory outcomes via LIPUS during either DO or fracture healing, this study was conducted using the same LIPUS protocol: the characteristics of the LIPUS signal were a frequency of 1.5 MHz, a signal repetition rate of 1 kHz, a pulse width of 200 μs, 117 mW of power, and an intensity of 30 mW/cm².

In this study, LIPUS stimulation was initiated during the distraction phase; however, the timing for application of LIPUS during DO is not standardized [26]. Most studies have preferred to conduct LIPUS during the consolidation phase [18, 19, 21, 23, 24, 36]. However, we hypothesized that LIPUS could play an important role in enhancing callus consolidation when applied during the distraction phase because active angiogenesis, which is closely related to mineralization in the distraction gap, occurs during the distraction phase [37]. Aronson demonstrated that regional blood flow at the corticotomy site increases 10-fold during the early distraction phase [38]. This increased blood flow decreases after surgery but remains up to 3 times greater than normal until 17 weeks postoperation. The increase in blood flow in this phase leads to an increase in hematopoietic function, differentiation of pluripotential cells, and active angiogenesis [37].

This study demonstrated that LIPUS enhanced callus consolidation, especially in sites with a thin covering of soft tissue (anterior and medial cortices). It is known that inadequate soft tissue coverage is closely related to late consolidation of the regenerated callus during DO; hence, calluses on these two cortices have a higher risk of delayed consolidation than those of the posterior and lateral cortices [7]. However, a thinner covering of soft tissue causes less attenuation of the LIPUS signal [23], which may increase the efficacy of LIPUS stimulation on the anterior and medial cortices during DO.

However, in terms of EFI, no significant positive effect was observed by LIPUS stimulation. We supposed that this finding was related to the lengthening over the nail procedure used in the current study. It is possible to remove the external fixator after achieving consolidation of only 2 of 4 cortices using the lengthening over the nail procedure, while it is safe to remove the external fixator after achieving complete consolidation of at least 3 of 4 cortices using the conventional Ilizarov procedure.

This study was limited by its retrospective nature. Also, the number of patients opting for LIPUS was not large enough due to the added cost of the therapy, limiting the number of patients recruited. These factors could affect the statistical significance. Nevertheless, all patients in this study were surgically treated by a single surgeon at a single institution with the same standardized LIPUS protocol. In addition, to the best of our knowledge, this is the first study evaluating the efficacy of LIPUS during DO by the technique of lengthening over the nail procedure.

LIPUS is a noninvasive and effective adjuvant therapy to enhance callus maturation during DO. It also enhances callus consolidation and may have a positive effect on appropriate callus shape and type.