Norian SRS Resorbable Cement for Augmentation of Internal Fixation of Hip Fractures

NORIAN SRS RESORBABLE CEMENT
FOR AUGMENTATION OF INTERNAL
FIXATION OF HIP FRACTURES

Stuart B. Goodman and Sune Larsson

Hip fractures are a serious health care problem. In 1994 in the United States alone, there were over 250,000 hip fractures, the majority of which occurred in women.1 Furthermore, the incidence of hip fractures is increasing due to the overall aging of the population as a whole and the increasing incidence of osteoporosis, which affects over 25 million Americans.² The health care expenditures for hip fractures are astounding: in 1995, these costs totaled approximately $8.7 billion in the United States. Hip fractures are also associated with significant morbidity and mortality. The current 6-month mortality is approximately 10 to 20%, despite modern medical treatment and aggressive rehabilitation. If the patient survives a hip fracture, there are major implications for quality of life thereafter.^1–8 Over two thirds of hip fracture patients do not return to their previous level of functioning. Forty percent of hip fracture patients require the use of a walking aid, and 60% can not perform their activities of daily living independently 1 year postoperatively. Cognitive level also diminishes after sustaining a hip fracture. This lack of cognitive function and independent living places a large social and financial burden on society.

Hip fractures are uniformly treated by optimization of the patient’s general medical condition, early internal fixation or prosthetic replacement of the fracture (within the first few days after injury), and comprehensive rehabilitation.⁵ The overall aim of treatment is to facilitate the patient’s early return to society in optimal fashion. Unfortunately, as the above statistics show, this goal has not been met for the majority of patients.

Internal fixation of hip fractures is predicated on the assumption that anatomical reduction of the fracture will lead to more normal hip biomechanics and therefore optimal function. Aside from the medical comorbidities associated with the elderly patient population, the main determinant of clinical outcome of hip fractures is the adequacy and maintenance of the fracture reduction. Hip fractures are notorious for impaction and displacement of the fracture fragments and loss of the reduction.^7–9 Although closed reduction techniques, the use of multiple screws (for femoral neck fractures), and the sliding hip screw and plate device (for intertrochanteric fractures) have revolutionized the treatment of hip fractures, poor clinical and radiological outcomes are common.^5,9 This is due in part to the disruption of the load-transmitting posteromedial calcar femorale and crushed cancellous bone in the femoral neck and intertrochanteric area postfracture. This places large axial, bending, and torsional loads on the fracture fragments and implant, leading to impaction, displacement, and implant failure.

Injectable materials that could supplement internal fixation in osteoporotic bone could potentially optimize the stability of hip fractures by resisting some of the loads mentioned above.^10–13 In this regard, Norian SRS is a biodegradable, nonexothermic bone cement with physical and chemical characteristics similar to the mineral phase of bone.¹⁴ When injected in the form of a paste, Norian SRS quickly hardens in situ, augmenting structural bone deficits in areas of compromised cancellous bone such as in the proximal femur.^15–17 Unlike polymethylmethacrylate, the material is biodegradable and able to undergo remodeling through normal biological processes. This chapter describes the clinical and experimental studies supporting the use of Norian SRS in the treatment of osteoporotic hip fractures. The general material and mechanical and biological properties of Norian SRS cement can be found elsewhere.^14–16

EXPERIMENTAL STUDIES WITH NORIAN SRS BONE CEMENT FOR HIP FRACTURES

Excessive shortening and altered hip biomechanics after fixation of femoral neck and unstable intertrochanteric fractures has been associated with poor functional results.5,9,18 Several biomechanical studies have been performed to assess the efficacy of Norian SRS combined with internal fixation in resisting physiological loads compared with the use of internal fixation alone.

FEMORAL NECK FRACTURES

Stankewich and colleagues¹⁹ studied the biomechanical effects of using Norian SRS to augment internally fixed, comminuted, femoral neck fractures. Pairs of fresh cadaveric proximal femora were harvested, stripped of soft tissue, and radiographed, then underwent dual-energy X-ray absorptiometry (DEXA) for bone mineral density and then were potted. A notch osteotomy was made at the midcervical level of the femoral neck using an oscillating saw; the fracture was completed using a materials testing machine (MTS, Indianapolis, IN). Thirty percent comminution at the inferior cortex of the fracture surface was created with an osteotome. The specimens were then randomly assigned into two groups of matched pairs that received internal fixation with cannulated screws with/without Norian SRS bone cement augmentation. The fractures were reduced anatomically (using direct vision and radiography) and three 7 mm diameter, 32 mm thread length cannulated cancellous bone screws were placed in an inverted triangle configuration. In the SRS treatment group, the drill holes and adjacent fracture were filled with SRS prior to insertion of each screw. After a 24-hour cement-curing period, the specimens were re-mounted on the testing machine (MTS), preconditioned, and tested using parameters simulating single-stance gait (1000 cycles of load to a peak compressive force of 611.6 N at 0.5 Hz). Stiffness of the construct was measured and then a quasistatic compressive load was applied at 10 mm/min until failure of fixation.

The fracture stiffness of the first loading cycle was found to be significantly greater in the SRS-augmented specimens compared to fractures fixed with screws alone (P<0.05). Furthermore, SRS-augmented specimens failed at significantly higher loads than the contralateral controls (P<0.01). On average, the addition of SRS improved the fixation strength of the fracture construct by approximately 170%. The relative improvement of fixation strength was not dependent on bone mineral density of the femoral neck.

This study suggests a potential biomechanical advantage in augmenting comminuted, internally fixed femoral neck fractures with Norian SRS.

INTERTROCHANTERIC FRACTURES

Biomechanical Study #1

In the first biomechanical study,²⁰ three-part intertrochanteric hip fractures were created in 10 matched pairs of osteoporotic human cadaveric femora. The femora first underwent radiographic analysis and dual energy X-ray absorptiometry (DEXA); only femoral pairs for which the density of the femoral neck was at least two standard deviations below that of young, gender-matched femora were chosen for inclusion in the study. The femora were stripped of soft tissue, potted, and two 45-degree strain gauge rosettes were attached to each femur along the femoral neck and immediately below the lesser trochanteric area. The femora were first loaded intact on a servohydraulic testing machine (MTS) using femoral head polymethylmethacrylate casts and parameters simulating single-leg stance (neutral flexion and 12.5-degree adduction). After preconditioning, a three-part intertrochanteric fracture was made by creating 2.2 mm stress risers at 6 mm intervals, including a wedged-shaped defect, and the femora were further loaded on the MTS machine. The femora were then anatomically reduced and fixed with a sliding hip screw device [Dynamic Hip Screw (DHS), Synthes, Paoli, PA] incorporating a 135-degree side plate and 4.5 mm bicortical screws. One of the randomly selected femora in each pair also received Norian SRS. Approximately 5 cc of SRS bone cement was injected into the proximal portion of the screw tract, prior to insertion of the sliding screw. After placement of all the internal fixation, an additional 10 cc of Norian SRS was injected and manually packed into the posteromedial defect left by the displaced lesser trochanter. After curing the cement for 12 hours, a uniaxial strain gauge was fixed to the side plate to measure tensile strains. Axial displacement of the sliding screw relative to the side plate was measured by attaching a linear variable differential transformer to the side plate during mechanical testing. The mechanical loading proceeded for 1000 cycles using 612 N compressive loads at 0.5 Hz and simulated single limb stance.

The addition of Norian SRS to the construct increased fracture stiffness (by approximately 165% after the first cycle and 75% after the thousandth cycle) and strength of fixation compared to nonaugmented contralateral controls (P<0.05). There was decreased sliding hip screw displacement in the SRS treatment group (0.07 mm versus 1.18 mm, P<0.05), although the amount of sliding was very small in both groups because of anatomic reduction of the fracture. Medial bone surface strain remained closer to the intact femur and side plate in the augmented group (P<0.05). Thus the adjunctive use of Norian SRS enhanced the stability of three-part intertrochanteric fractures in osteoporotic femora.

Biomechanical Study #2

A second biomechanical study was performed to evaluate the use of Norian SRS augmentation of unstable three-part intertrochanteric femoral fracture fixation using a specific technique that could be applied at surgery.21 Two treatment groups were evaluated; namely (1) fixation of the fracture with a DHS alone and (2) fixation with a DHS and Norian SRS cement augmentation of the intertrochanteric fracture line and posteromedial lesser trochanteric region of the proximal femur. Paired, human cadaveric femora were first prescreened for anatomical deformities and bone mineral density (BMD) by radiographic examination and DEXA, respectively, prior to admission to the study. Six pairs of femora were subsequently entered in the study, cleaned of soft tissue, and potted in aluminum cylinders. An acrylic acetabular cup was molded using individual femoral heads to simulate in vivo loading conditions around the femoral head. The axial load was applied vertically to the femoral head with the shaft oriented in neutral flexion and 12.5 degrees of adduction to simulate the loads experienced in single-leg stance. A three-part unstable intertrochanteric fracture was created by generating stress risers along the proposed fracture lines and loading on an MTS machine. The two major proximal and distal fragments were reduced and fixed with the DHS. The lesser trochanteric fragment was not reduced. Prior to placement of the side plate in the SRS-treated femora, approximately 10 cc of SRS was injected into the intertrochanteric fracture area with a 12-gauge needle and syringe under radiographic image intensification. The side plate was applied, and all screws except the most proximal one were then inserted in the side plate. Using the proximal screw hole in the side plate, the posteromedial lesser trochanteric defect was then filled with 10 to 20 cc of Norian SRS using a 12-gauge curved needle and syringe under image intensification. The contralateral control femora were treated with a DHS alone without SRS. A linear variable differential transducer (LVDT) was then attached to the side plate using a custom fixture. After mechanical preconditioning, each femur was loaded to 1650 N (2.5 × body weight) for 10,000 cycles to simulate postoperative load transmission across the fracture construct during normal gait. The load was further increased successively by one body weight for another 10,000 cycles until failure of the construct. Fixation was evaluated by measuring the amount of sliding of the lag screw of the DHS (shortening) and stiffness of the overall fracture construct (stability).

The mean sliding of the lag screw after the first 10,000 cycles was 0.73 mm (range 0.07 to 2.07) for the SRS cement-treated specimens and 18.76 mm (range 2.89 to 43.27) for the control specimens (P= 0.026). First and 20th cycle stiffness was significantly greater in the SRS cement specimens (P= 0.035 and P= 0.026, respectively). Thus, the addition of Norian SRS was associated with less femoral shortening and increased stability of three-part intertrochanteric fractures. Decreased proximal femoral shortening would be associated with improved hip biomechanics.

CLINICAL STUDIES WITH NORIAN SRS BONE CEMENT FOR HIP FRACTURES

As most patients who suffer a hip fracture are elderly, the surgical procedure should aim at providing adequate stability in the fracture area so that unrestricted weight bearing can be allowed directly after surgery. Restricted weight bearing is often difficult in elderly patients due to reduced muscle strength in the upper extremities, which makes efficient unloading of an injured hip during ambulation tiresome. If a stable fracture reconstruction is not achieved, rehabilitation will be prolonged and the risk for secondary fracture displacement will increase.

Femoral Neck Fractures

The main surgical options include either internal fixation or prosthetic replacement. The complications after internal fixation of femoral neck fractures can usually be divided into early mechanical loosening, nonunion, or avascular necrosis due to disturbed circulation to the femoral head. The revision rate following internal fixation of displaced femoral neck fractures is 20 to 35%⁸; complications after undisplaced femoral neck fractures are less than 10%. Even though primary circulatory disturbance of the femoral head at the time of injury is the strongest outcome predictor after internal fixation of displaced femoral neck fractures, fracture stability also correlates strongly with uneventful healing.²² Unfortunately the bone is usually osteoporotic, which makes it difficult for screws to achieve a good grip in the femoral head.²³ Furthermore, comminution at the fracture site, most common in the posterior aspect of the fracture, adds to the difficulties when aiming for stable fracture fixation. As shown by Stankewich et al,¹⁹ augmentation with calcium-phosphate cement (Norian SRS) of experimental femoral neck fractures provided enhanced stability when loaded in a material testing machine and compared with fractures fixed with cannulated screws alone. It therefore seemed of great interest to find out whether such an enhancement with calcium-phosphate cement could provide better stability and improved clinical outcome. Even if prosthetic replacement provided a good outcome in the short term, few would argue that successful internal fixation followed by uneventful healing would be a better option for the patient who has sustained a femoral neck fracture.

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