First introduced in the early twentieth century [1
], distraction osteogenesis is a principle in which new bone (called the regenerate) develops in an area subjected to gradual tension. This technique was further developed and popularized by Professor Gavril Ilizarov during the 1950s for limb lengthening and reconstruction [2
]. Distraction osteogenesis is now used worldwide for limb reconstruction particularly in cases of bone tumors, congenital deformities, bone defects, and osteomyelitis [3
]. A minimally invasive, low-energy osteotomy is performed to fracture a bone into two segments. Usually, an external fixator is applied through percutaneously placed transosseus pins and/or wires that are connected to external scaffolding. The external fixator is used to stabilize the fragments and manipulate them to achieve lengthening or deformity correction.
External fixation rather than internal fixation is used for several reasons. The greatest advantage of external fixators is that they allow for control of movement of the bone fragments in multiple planes [4
]. This postoperative adjustability is not possible using internal fixation. Additionally, external fixators are ideal for cases where soft tissue or bone is infected, tenuous, or poorly vascularized [5
However, external fixation is not without its drawbacks. While there is an advantage to be able to place pins or wires through healthy-appearing skin, this ultimately creates a communicating tract between the skin and bone, increasing the potential for the development of pin tract infection which can lead to osteomyelitis [6
]. Prolonged periods of time in an external fixator can lead to multiple problems including osteopenia [8
], an increased rate of persistent pain [9
], and a considerable psychological burden [10
]. The importance of this issue has even been recognized by Professor Ilizarov who wrote that “leaving the apparatus on for longer than necessary is as harmful as removing the fixator too early” [12
]. Orthopedic surgeons have taken direct aim at shortening the length of time required in the external fixator by creating new limb lengthening and deformity correction techniques, such as lengthening over nail and lengthening and then nailing [6
Determining the right time to remove the frame remains a challenge, and proper timing is extremely important to prevent regenerate refracture or deformation resulting in persistent deformity (see Fig. ). Many argue that this is the most difficult and important decision the surgeon must make in limb lengthening and deformity correction. Several studies have attempted to model formulas to help predict the length of time one should remain in an external fixator. For example, Dinah [14
] found after reviewing 27 tibia lengthenings in 24 patients that the required time in a frame could be estimated to be 54 days per desired centimeter of total limb lengthening centimeters on average for all patients. Unfortunately, formulas such as this do not work in many cases because bone healing is dependent on both biological and mechanical factors such as age, underlying pathology, mechanical load on and stiffness of the fixation device [15
]. For this reason, all patients must be monitored individually for the assessment of progress of their bone formation.
Fig. 1 AP and lateral digital radiographs of a 70-year-old woman who underwent knee fusion and femoral lengthening for a bone defect resulting from multiple failures of total knee arthroplasty. She presented with a collapsed regenerate 7 weeks after (more ...)
Most surgeons balance qualitative and subjective assessments of bone healing against pressure from the patient (and the same surgeon) to have the frame removed. Many surgeons take into account variables such as whether the patient still has pain with weight bearing, examining gait with the external fixator, and/or dynamizing the frame for a period of time with observation. Even so, the main tool for healing assessment in clinical practice today is standard radiography in two planes. This standard comes from a study by Fischgrund et al. [15
] who reported a low fracture rate of 3% when using guidelines that required three of four cortices in the anteroposterior and lateral radiographs to be continuous and at least 2 mm thick in order to have the external fixator removed. While using standard radiography is cheap and quick, the method of identifying three of four cortices does not produce reliable results. Anand et al. [16
] found inter-observer agreement to be less than 50% between all involved orthopedic surgeons, suggesting that the assessment of bone healing by radiographs is subjective. Starr et al. [17
] found that the decision to remove an external fixator based on radiographic assessment alone resulted in intra-observer and inter-observer variability moderately above chance. This high variability as to when surgeons find it appropriate to remove an external fixator may contribute, along with other factors (such as primary diagnosis), to the large cited rate of refracture after frame removal (3% to 50%) [18
There is a need for a quick, inexpensive, and simple quantitative method to help orthopedic surgeons with determining when is it is appropriate to remove the frame after distraction osteogenesis. Direct stiffness measurements of the regenerate to assess healing using strain gauges attached to the fixator are often cumbersome and require the removal of the fixator. In addition, the equipment used for these direct tests are expensive and may not be available to all practices. Indirect methods are therefore of great interest to the orthopedic community. The literature has several individual reports of groups investigating methods such as dual-energy X-ray absorptiometry (DEXA) scan, ultrasound (US), quantitative computer tomography (QCT), and plain radiographs for this purpose. However, there is no current review in the literature. We therefore believe that a paper summarizing the literature on assessing bone healing after distraction osteogenesis would be useful.