Implantable strain sensor to monitor fracture healing with standard radiography

Abstract

Current orthopaedic clinical methods do not provide an objective measure of fracture healing or weight bearing for lower extremity fractures. The following report describes a novel approach involving insitu strain sensors to objectively measure fracture healing. The sensor uses a cantilevered indicator pin that responds to plate bending and an internal scale to demonstrate changes in the pin position on plain film radiographs. The long lever arm amplifies pin movement compared to interfragmentary motion, and the scale enables more accurate measurement of position changes. Testing with a human cadaver comminuted metaphyseal tibia fracture specimen demonstrated over 2.25 mm of reproducible sensor displacement on radiographs with as little as 100 N of axial compressive loading. Finite element simulations determined that pin displacement decreases as the fracture callus stiffens and that pin motion is linearly related to the strain in the callus. These results indicate that an implanted strain sensor is an effective tool to help assess bone healing after internal fixation and could provide an objective clinical measure for return to weight bearing.While a variety of fracture stabilization therapies are available to surgeons for internal fixation, the inability to directly evaluate healing remains a limiting factor when assessing patient recovery and return to a pre-injury activity level. Tibial fractures are the most common long-bone fracture (36.7% of all long-bone fractures in adults) and also happen to be the most common site of long-bone nonunion 1 . Patients with nonunion are more likely to have additional fractures during follow-up, to require various types of in-patient and surgical care (including amputation), to be prescribed pain medications (especially strong opioids), and to use more outpatient physical therapy than those with proper bony union 2 . Median total costs of care for patients with nonunion were found to be more than double those without ($25,556 vs. $11,686 according to 2006 data) 2 . To help prevent complications such as non-union or malunion, refracture and implant failure, physicians often limit weight bearing for an extended time, often 12 weeks or longer, to allow for adequate bone growth 3 . While premature weight bearing can increase the rate of complications, unnecessarily delaying weight bearing results in productivity loss with indirect costs from lost wages and places additional burdens on the healthcare system. Identification of post-operative complications (non-union, infection, implant loosening, etc.) throughout the recovery process, while critical to effective treatment, is often difficult with existing internal fixation methods.Current clinical methods of monitoring bone healing and identifying complications following an open-reduction and internal fixation (ORIF) are typically limited to a combination of symptomatic, physical examination, and radiographic findings. Interpretation is subjective, influenced both by the patient's candid description of symptoms and by the treati...

An implanted pH sensor read using radiography

“…We previously reported on a mechanical sensor with similar pin-and-scale readout to measure orthopedic implant bending under load to track fracture healing. 25 The sensor described here provides complementary local chemical information relevant for monitoring implantassociated infections.…”

Effects of short‐ and long‐term administration of nonsteroidal anti‐inflammatory drugs on osteotomy healing in dogs

“…However, the difference between least squares mean for treatment and the 95% CI calculated (Table ) for each variable that did not reach statistical difference between groups did not justify that conclusions be changed. In human medicine, inability to perform antemortem biomechanical testing as well as limitations of radiography as a sole gauge of fracture healing has led to the development of more objective methods of fracture evaluation, such as implantation of a strain sensor across a healing fracture . This device was created to be used adjunctively with radiographic union assessment for evaluation of fracture healing.…”

“…In human medicine, inability to perform antemortem biomechanical testing as well as limitations of radiography as a sole gauge of fracture healing has led to the development of more objective methods of fracture evaluation, such as implantation of a strain sensor across a healing fracture. 49 This device was created to be used adjunctively with radiographic union assessment for evaluation of fracture healing.…”

Measuring Orthopedic Plate Strain to Track Bone Healing Using a Fluidic Sensor Read via Plain Radiography

“…[15], [21] Previously, we made a plate attachment with a tungsten pin that moved against a scale to quantitatively indicate plate bending via plain radiography. [24] However, the 12 cm long pin was surgically cumbersome, and reducing pin length would require a protruding scale and much smaller displacement. To address this concern, herein we developed a smaller device which uses hydraulic gain to amplify the signal from plate bending and increase precision for tracking healing bone.…”