Biomechanical Evaluation of Peri- and Interprosthetic Fractures of the Femur

Research Update on Stress Riser Fractures

Stress fractures are fatigue-induced fractures which are caused by repetitive force, often from overuse. They are well-established and frequently encountered in the field of orthopedics. Stress fractures occur in the bone because of low-bone strength and high chronic mechanical stress placed on the bone. Stress riser fractures are also stress fractures that occur because of the presence of cortical defects (holes), changes in stiffness, sharp corners, and cracks (fracture lines). Periprosthetic or peri-implant fractures are good examples of stress riser fractures that occur in regions where stress forces are higher than those in the surrounding material. Most stress riser fractures are related to technical errors (iatrogenic causes) and are difficult to manage. It is possible and more effective to prevent the creation of stress riser fractures through better surgical techniques. The proper terminology for stress fractures, stress riser fractures, periprosthetic fractures, peri-implant fractures, interprosthetic fractures, and interimplant fractures is discussed. This review of the current state of knowledge, diagnosis, treatment, and prevention of stress riser fractures is based on clinical evidence and recent literature.

Interprosthetic Femoral Fractures Surgical Treatment in Geriatric Patients

Background

Total hip (THA) and total knee arthroplasty (TKA) are becoming an increasingly standard procedure in the whole world. In conjunction with an aging population and increased prevalence of osteoporosis, proper management of periprosthetic, and interprosthetic fractures is of great interest to orthopedic surgeons. This study aims to report the clinical and radiographic outcomes, complications and reoperations of IFFs in geriatric patients.

Methods

A retrospective single-institution case series study was conducted. Between 2011 and 2019, 83 patients underwent surgical treatment for periprosthetic femoral fractures. Thirteen fractures were identified as IFFs. Patient demographics and comorbidities were collected preoperatively, and fractures were classified with the Vancouver and AO unified classification system (AO-UCS).

Results

We included 12 patients (13 hips) with IFFs (AO-UCS type IV.3 B (2/13) type IV.3 C (3/13), type IV.3 D (8/13)). The average patient age was 86.54 (range, 79-89) years. There were 10 females and 2 males. Perioperative morbidity has been identified in 10 of the 12 patients, and the 3-month and 1-year mortality were reported in 2 and 3 patients, respectively. Cerclage cables were used in 9 of 12 patients. One of 12 patients showed a local complication, with no documented implant failure or revision. Patients achieved complete union and returned to their preoperative ambulatory status, and full weight-bearing at an average of 5 (range, 2 to 7) months later.

Conclusion

Management of IFF can be challenging because these fractures require extensive surgical expertise. Locking plate seems to be a valuable treatment option for geriatric patients with IFFs. Despite the complexity of this type of fracture, the overall complication and revision rate, as well as the radiographic outcome are good to excellent.

Level of Evidence

Level III, Therapeutic study.

Periprosthetic fracture fixation of the femur following total hip arthroplasty: A review of biomechanical testing - Part II

BACKGROUND

Periprosthetic femoral fracture is a severe complication of total hip arthroplasty. A previous review published in 2011 summarised the biomechanical studies regarding periprosthetic femoral fracture and its fixation techniques. Since then, there have been several commercially available fracture plates designed specifically for the treatment of these fractures. However, several clinical studies still report failure of fixation treatments used for these fractures.

METHODS

The current literature on biomechanical models of periprosthetic femoral fracture fixation since 2010 to present is reviewed. The methodologies involved in the experimental and computational studies of periprosthetic femoral fracture fixation are described and compared with particular focus on the recent developments.

FINDINGS

Several issues raised in the previous review paper have been addressed by current studies; such as validating computational results with experimental data. Current experimental studies are more sophisticated in design. Computational studies have been useful in studying fixation methods or conditions (such as bone healing) that are difficult to study in vivo or in vitro. However, a few issues still remain and are highlighted.

INTERPRETATION

The increased use of computational studies in investigating periprosthetic femoral fracture fixation techniques has proven valuable. Existing protocols for testing periprosthetic femoral fracture fixation need to be standardised in order to make more direct and conclusive comparisons between studies. A consensus on the 'optimum' treatment method for periprosthetic femoral fracture fixation needs to be achieved.

Optimal plate fixation of distal femoral fractures in the presence of a well fixed cemented hip arthroplasty femoral stem

INTRODUCTION

The requirement for lateral plate fixation in the presence of a hip arthroplasty stem is increasing, yet the optimal plate construct necessary to reduce inter-prosthetic strain and prevent subsequent fractures is unknown. Therefore, the purpose of this study was to determine the optimal position of a lateral femoral plate in relation to a femoral stem.

METHODS

We used 4th Generation Composite Sawbones^®, a collarless polished cemented femoral stem and a broad Peri-loc™ plate with varying overlap of the stem. Each construct was subjected to torsion, axial compression and 3-point lateral bending, with the strain measured at the tip of the femoral stem and the force-to-failure recorded.

RESULTS

In all three loading directions a plate, which extended at least two shaft diameters proximal to the tip of the stem, experienced the lowest strain ( p < 0.001). Constructs with no overlap, and particularly those with a small gap between the tip of the stem and the proximal extent of the plate experienced the highest strain ( p < 0.001). This high strain resulted in lower forces required to cause fracture in the clinically significant scenarios of femoral torsion and lateral bending.

CONCLUSIONS

Optimal plate fixation is achieved by extending a lateral femoral plate at least two shaft diameters proximal to the tip of the stem and using a spread of proximal screws. Distal fixation increases the strain experienced at the tip of the stem and higher strains are noted the smaller the gap between the proximal extent of the femoral plate and the tip of stem.

Interprosthetic Fractures of the Femur

As the rate of hip and knee arthroplasty procedures increases, so will the rate of interprosthetic fractures. Several factors, including bone quality, bone quantity, and stability of the prosthetic components, play a role in determining the appropriate operative treatment. Patients with stable components should undergo reduction and internal fixation, while patients with loose components should undergo either revision arthroplasty, with or without additional fixation, or conversion to total femur replacement. Despite implant and technique advances, complications remain frequent. [Orthopedics. 2018; 41(1):e1-e7.].

Validation of a new classification system for interprosthetic femoral fractures

Interprosthetic femoral fracture (IFF) incidence is gradually increasing as the population is progressively ageing. However, treatment remains challenging due to several contributing factors, such as poor bone quality, patient comorbidities, small interprosthetic fragment, and prostheses instability. An effective and specific classification system is essential to optimize treatment management, therefore diminishing complication rates. This study aims to validate a previously described classification system for interprosthetic femoral fractures.

Periprosthetic Femoral Fractures in the Emergency Department: What the Orthopedic Surgeon Wants to Know

Femoral fracture in the setting of a hip arthroplasty is an increasingly common complication encountered in the emergency department (ED). Diagnosis and management of periprosthetic fractures are complicated, and orthopedic surgeons rely on imaging findings to guide the appropriate management approach to the injury. Delay in identification and appropriate definitive management of periprosthetic fractures is associated with high morbidity and mortality. At present, the Vancouver classification system for periprosthetic hip fractures is the most common classification system used by orthopedic surgeons. It relies on three radiographic criteria-fracture location, prosthesis stability, and quality of the femoral bone stock-to characterize these fractures and to help guide management decisions. Familiarly with the Vancouver classification system allows radiologists to both recognize and communicate the most clinically relevant imaging findings to the treating orthopedic surgeon. This article reviews the imaging workup for hip pain in patients with a femoral prosthesis, risk factors for periprosthetic fracture, and the expected normal appearance of the most commonly encountered types of femoral prostheses. Fracture terminology and the Vancouver classification system are reviewed in a simplified algorithm with emphasis on the most common patterns of periprosthetic fractures, the radiologic determinants of prosthesis stability and bone quality, and the management implications of these imaging findings. Finally, multiple instructive clinical cases are used to demonstrate critical application of the classification system and to highlight the clinical implications of the imaging findings. ^©RSNA, 2017.

Management of Interprosthetic Femur Fractures

Femoral fractures between a total hip arthroplasty prosthesis and total knee arthroplasty prosthesis, also called interprosthetic fractures, are challenging clinical problems. The number of patients who have undergone ipsilateral primary or revision joint arthroplasty procedures in both the hip and the knee continues to rise, and the number of interprosthetic fractures is increasing, as well. The growing body of biomechanical and clinical literature on interprosthetic fractures reflects the increased frequency of and interest in these injuries. Similar to the management of periprosthetic fractures, the management of interprosthetic fractures depends on the location of the fracture, the stability of the implant, and the ability to achieve stable fracture fixation. These factors are the basis of recently described classification systems and treatment strategies. In patients with stable implants, fracture fixation alone is performed. When the implant is loose, both revision arthroplasty and fracture fixation may be required to provide stability of the limb.

[Periprosthetic and interimplant femoral fractures: Biomechanical analysis]

BACKGROUND

The requirements for treatment of femoral fractures are increasing with the rising age of the patients. The aim of this study was to evaluate femoral stiffness and the fracture risk after inserting different implants and implant combinations.

METHODS

A total of 48 cadaveric femors were harvested and 8 groups were randomized on the basis of the bone mineral density (BMD). Different osteosyntheses following prosthetic stem implantation (hip and knee prostheses) were performed and compared with native femurs. All specimens were biomechanically tested in a four-point bending until fracture.

RESULTS

The insertion of a femoral stem decreases the stiffness by approximately 33%. Ipsilateral retrograde nailing reduces the force to failure even more. This instable situation can be stabilized by a lateral locking compression plate. The most stable situation occurred in the presence of two cemented stems.

CONCLUSION

Taken together these results clearly indicate that a hip prosthesis significantly weakens the femur, whereas two stems produce the most stable situation at all. The situation of a hip prosthesis and an retrograde nail should be avoided or covered by a bridging-osteosynthesis. In clinical practice an extramedullary fixation technique for distal femoral fractures should be preferred.

Periprosthetic fracture fixation in osteoporotic bone

Fixation techniques of periprosthetic fractures are far from ideal although the number of this entity is rising. The presence of an intramedullary implant generates its own fracture characteristics since stiffness is altered along the bone shaft and certain implant combinations affect load resistance of the bone. Influencing factors are cement fixation of the implant, intramedullary locking and extramedullary or intramedullary localization of the implant and the cortical thickness of the surrounding bone. Cerclage wires are ideally suited to fix radially displaced fragments around an intramedullary implant but they are susceptible to axial and torsional load. Screws should be added if these forces have to be neutralized. Stability of the screw fixation itself can be enhanced by embracement configuration around the intramedullary implant. Poor bone stock quality, often being present in metaphyseal areas limits screw fixation. Cement augmentation is an attractive option in this field to enhance screw purchase.

Incidence of and risk factors for femoral fractures in the gap between hip and knee implants

PURPOSE

The purpose of this study was to analyse the incidence of interprosthetic femoral fractures and describe risk factors for them.

METHODS

Between 2009 and 2015, we selected patients who were carrying two implants (hip and knee) in the same femur. We collected demographic and clinical data and performed a radiological evaluation to analyse the gap between implants-the femoral canal area and total femoral area-in the axial plane. We defined interprosthetic fracture as that corresponding to a Vancouver type C fracture and types 1 and 2 according to the Su classification.

RESULTS

We studied 68 patients who had total knee arthroplasty (TKA), and 44 patients who had total hip arthroplasty (THA); 24 patients an intramedullary nail. We found six interprosthetic fractures (8.8 %), all in patients with a non-cemented THA. There was a tendency towards statistical difference (p = 0.08). Patients with an additional implant at the proximal femur were statistically less likely to have an interprosthetic fracture (p = 0.04). In radiological results, we found more interprosthetic fractures in patients who had an increased femoral canal area in the axial plane just distal to the tip of the hip implant.

CONCLUSIONS

Identifying risk factors for this specific type of fracture may facilitate their prevention. Better implant stability and the presence of a gap between stems in a lower canal zone appear to hinder the occurrence of interprosthetic fractures.

Interposition sleeve as treatment option for interprosthetic fractures of the femur: a biomechanical in vitro assessment

PURPOSE

The number of patients having hip and knee arthroplasties on the ipsilateral leg is going to rise. In this regard, the prevalence of interprosthetic femoral fractures is going to increase further. The treatment of these fractures is difficult and sometimes it is impossible to perform an osteosynthesis because of worse bone quality. The goal of this study was to investigate the use of an interposition sleeve as an alternative treatment option for interprosthetic fractures with major bone loss.

METHODS

Six human cadaveric femurs were instrumented using cemented hip- and knee prosthesis. Interprosthetic fractures were induced during a four-point-bending test and then treated using the interposition sleeve. Afterwards the constructs were tested using the four-point-bending test again.

RESULTS

Load-to-failure of the construct before fracturing was significantly higher than after treatment with the interposition sleeve (10681 N vs. 5083 N; p = 0.002). The failure mechanism of the femurs with the interposition sleeve was plastic deformation of the hip or knee prosthesis. The interposition sleeve did not fail in any specimen.

CONCLUSION

The interposition sleeve is a valuable treatment option for interprosthetic fractures in situations in which osteosynthesis is impossible or insecure due to major bone defects. However, fracture healing should be preferred whenever possible.

[Ipsilateral THA after stemmed TKA: Risk of interprosthetic fracture?]

BACKGROUND

There is an increasing number of patients who have undergone stemmed total knee arthroplasty and total hip arthroplasty in the same leg. These patients may be at increased risk of periprosthetic and interprostethic fracture. Etiology and the potential therapy strategies are not well represented in the current literature.

OBJECTIVES

Determination of the risk factors for interprosthetic fractures and possible prevention.

MATERIALS AND METHODS

We performed a review of the literature and additionally a survey among experts (members of the German Association for Arthroplasty [AE]) to investigate the risk and the necessity for a preventive internal fixation.

RESULTS

There are only a few biomechanical studies. The interprosthetic distance seems to have little influence on the fracture risk, but the thickness of the cortex and the cortical area at the diaphysis seems to be important. The value of a bridging osteosynthesis remains uncertain. Ninety experts took part in the survey. The risk of fracture risk was estimated to be only slight to medium. Opinions regarding the necessity of preventive internal fixation were heterogeneous.

CONCLUSIONS

The indication for preventive internal fixation could be derived neither from the literature nor from the survey of experts. The thickness of the cortex and co-morbidities (osteoporosis, tendency to fall, and medication) seem to be more important than the interprosthetic distance.

Orthogonal femoral plating: a biomechanical study with implications for interprosthetic fractures

Objectives

This study tests the biomechanical properties of adjacent locked plate constructs in a femur model using Sawbones. Previous studies have described biomechanical behaviour related to inter-device distances. We hypothesise that a smaller lateral inter-plate distance will result in a biomechanically stronger construct, and that addition of an anterior plate will increase the overall strength of the construct.

Methods

Sawbones were plated laterally with two large-fragment locking compression plates with inter-plate distances of 10 mm or 1 mm. Small-fragment locking compression plates of 7-hole, 9-hole, and 11-hole sizes were placed anteriorly to span the inter-plate distance. Four-point bend loading was applied, and the moment required to displace the constructs by 10 mm was recorded.

Results

We found that a 1 mm inter-plate distance supported greater moments than a 10 mm distance in constructs with only lateral plates. Moments supported after the addition of a 9- or 11-hole anterior plate were greater for both 10 mm and 1 mm inter-plate distance, with the 11-hole anterior plate supporting a greater moment than a 9-hole plate. Femurs with a 7-hole anterior plate fractured regardless of lateral inter-plate distance size.

Conclusion

This suggests that the optimal plate configuration is to minimise lateral inter-plate distance and protect it with an anterior plate longer than seven holes.

Cite this article: Bone Joint Res 2015;4:23–8.

Periprosthetic femoral fractures after hemiarthroplasty. An analysis of 17 cases

PURPOSE

To describe the characteristics of patients with periprosthetic femoral fractures after hemiarthroplasty and analyze their treatment.

MATERIAL AND METHOD

An observational, longitudinal, retrospective study was conducted on a series of 17 patients with periprosthetic femoral fractures after hip hemiarthroplasty. Fourteen fractures were treated surgically. The characteristics of patients, fractures and treatment outcomes in terms of complications, mortality and functionality were analyzed.

RESULTS

The large majority (82%) of patients were women, the mean age was 86 years and with an ASA index of 3 or 4 in 15 patients. Ten fractures were type B. There were 8 general complications, one deep infection, one mobilization of a non-exchanged hemiarthroplasty, and 2 non-unions. There were 85% consolidated fractures, and only 5 patients recovered the same function prior to the injury. At the time of the study 9 patients had died (53%).

DISCUSSION

Periprosthetic femoral fractures after hemiarthroplasty will increase in the coming years and their treatment is difficult.

CONCLUSION

Periprosthetic femoral fractures after hemiarthroplasty are more common in women around 90 years-old, and usually occur in patients with significant morbidity. Although the Vancouver classification is reliable, simple and reproducible, it is only a guide to decide on the best treatment in a patient often fragile. The preoperative planning is essential when deciding a surgical treatment.

Interprosthetic femoral fractures: proposed new classification system and treatment algorithm

Interprosthetic femoral fracture is a rare and challenging fragility fracture issue. Due to aging of the population, the incidence of this type of fracture is gradually and constantly increasing. There is no complete and specific interprosthetic femoral fracture classification system that indicates treatment and prognosis in the literature. The aim of the present study was to describe a new classification system for interprosthetic femoral fractures, and to present a case series and a treatment algorithm derived from the current evidence in the literature.

The role of inter-prosthetic distance, cortical thickness and bone mineral density in the development of inter-prosthetic fractures of the femur

It is becoming increasingly common for a patient to have ipsilateral hip and knee replacements. The inter-prosthetic (IP) distance, the distance between the tips of hip and knee prostheses, has been thought to be associated with an increased risk of IP fracture. Small gap distances are generally assumed to act as stress risers, although there is no real biomechanical evidence to support this.

The purpose of this study was to evaluate the influence of IP distance, cortical thickness and bone mineral density on the likelihood of an IP femoral fracture.

A total of 18 human femur specimens were randomised into three groups by bone density and cortical thickness. For each group, a defined IP distance of 35 mm, 80 mm or 160 mm was created by choosing the appropriate lengths of component. The maximum fracture strength was determined using a four-point bending test.

The fracture force of all three groups was similar (p = 0.498). There was a highly significant correlation between the cortical area and the fracture strength (r = 0.804, p <  0.001), whereas bone density showed no influence.

This study suggests that the IP distance has little influence on fracture strength in IP femoral fractures: the thickness of the cortex seems to be the decisive factor.

Cite this article: Bone Joint J 2014;96-B:1378–84.

Interprosthetic femoral fractures treated with locking plate

PURPOSE

Interprosthetic fractures are challenging to manage. Although treatment of femoral fractures around a single implant has been described, there is little literature for treatment of interprosthetic femoral fractures. This study analyses the management and outcomes of 15 patients with interprosthetic femoral fractures treated with locking plates.

METHODS

A retrospective chart review was conducted of 17 patients with interprosthetic femur fracture treated with locking plates from 2002 to 2013. Patient demographics and comorbidities were collected. Preoperatively, patients were classified with the Vancouver or Su classification system. Intraoperative use of bone graft and/or cerclage cables was also examined. Clinical and radiographic outcomes were evaluated for union, time to full weight bearing, return to preinjury level of activity, and pain assessed with visual analog scale (VAS).

RESULTS

There were 15 patients with interprosthetic fractures meeting criteria for this study. Average patient age was 80.53 (range, 61-92) years. Bone grafting was used in 23.5% (four of 17) and cerclage cables in 29.4% (five of 17). Patients achieved complete union and return to full weight bearing an average of 4.02 (range, two to six) months later. Average VAS pain score was 1.00 (range, zero to six). All patients returned to their preoperative ambulatory status.

CONCLUSION

Locking plates could achieve satisfactory results for interprosthetic fractures. Considering an individual's fracture type, bone quality and protheses to determine the appropriate plate length and optional use of cerclage and/or bone graft was essential. In this limited sample size, interprosthetic fractures occurred at similar rates at the supracondylar region and diaphysis.

Femoral stress and strain changes post-hip, -knee and -ipsilateral hip/knee arthroplasties: a finite element analysis

OBJECTIVE

To identify the optimal ratio of free femur for minimizing the risks of periprosthetic fracture.

METHODS

Three dimensional models of the femur with hip and knee stem elongation were constructed. With the distal femoral condylar surface fixed in a three dimensional model, the femoral head loading was performed according to the methods described by Huiskes and van Rietbergen in the models of hip replacement, knee replacement with or without hip stem or knee stem elongation. The maximum principal stress (MPS) and maximum principal elastic strain (MPES) of the femur were recorded and their relationships to the free femur ratio were analyzed using Pearson's correlation analysis.

RESULTS

There were no obvious changes in MPS and MPES with hip stem elongation from 100 to 180 mm. In ipsilateral hip and knee replacement, the MPS and MPES had a tendency to decrease with knee and hip stem elongation. The MPS and MPES were mainly located in the anterior medial side of the middle to distal femur post-hip replacement and distalized with stem elongation. When the knee stem had been elongated more than 120 mm, the stress and strain concentrated strongly in the middle of the femoral shaft. There was a positive correlation between MPS and MPES to the free femur ratio (P < 0.01); however, no optimal ratio of free femur that would minimize the risks of periprosthetic fracture was identified.

CONCLUSION

Positive correlations were found between implant free femur and stress and strain changes in total knee arthroplasty, total hip arthroplasty and ipsilateral hip and knee replacement.

Interprosthetic femoral fractures-A challenge of treatment. A systematic review of the literature

The success of prosthetic surgery has led to an increase in the percentage of the population having more than one prosthetic implant. This, combined with an increase in the average life expectancy and functional requirements for the elderly, has led to a higher incidence of periprosthetic and interprosthetic fractures. More precisely, the femoral shaft is compressed between two ipsilateral implants with most of these fractures being located on the supracondylar femoral shaft. Their treatment is not only technically demanding and challenging, but can also be associated with serious complications. Treatment must be determined and assessed according to the type of fracture, the stability of the prosthesis, the bone quality and the general condition of the patient. There is little information in the literature about this type of injury: there are several published case reports detailing unconventional solutions and the case studies presented are limited and not significant. This review aims to provide an updated and comprehensive list of diagnostic and therapeutic protocols accepted today, while recognising that these protocols are being continuously updated according to experience gained.

Minimally invasive fixation of type B and C interprosthetic femoral fractures

INTRODUCTION

Interprosthetic femoral fractures are rare and raise unresolved treatment issues such as the length of the fixation material that best prevents secondary fractures. Awareness of the advantages of locked-plate fixation via a minimally invasive approach remains limited, despite the potential of this method for improving success rates.

HYPOTHESIS

Femur-spanning (from the trochanters to the condyles) locked-plate fixation via a minimally invasive approach provides high healing rates with no secondary fractures.

MATERIALS AND METHODS

From January 2004 to May 2011, all eight patients seen for interprosthetic fractures were treated with minimally invasive locked-plate fixation. Mean time since hip arthroplasty was 47.5 months and mean time since knee arthroplasty was 72.6 months. There were 12 standard primary prostheses and four revision prostheses; 11 prostheses were cemented and a single prosthesis showed femoral loosening. Classification about the hip prostheses was Vancouver B in one patient and Vancouver C in seven patients; about the knee prosthesis, the fracture was SoFCOT B in three patients and SOFCOT C in five patients, and a single fracture was SoFCOT D. Minimally invasive locking-plate fixation was performed in all eight patients, with installation on a traction table in seven patients.

RESULTS

Healing was obtained in all eight patients, after a mean of 14 weeks (range, 12-16 weeks). One patient had malalignment with more than 5° of varus. There were no general or infectious complications. One patient died, 32 months after surgery. The mean Parker-Palmer mobility score decreased from 6.2 pre-operatively to 2.5 at last follow-up. Early construct failure after 3 weeks in one patient required surgical revision. There was no change in implant fixation at last follow-up. No secondary fractures were recorded.

DISCUSSION

In patients with type B or C interprosthetic fractures, femur-spanning fixation not only avoids complications related to altered bone stock and presence of prosthetic material, but also decreases the risk of secondary fractures by eliminating stress riser zones. The minimally invasive option enhances healing by preserving the fracture haematoma. Thus, healing was obtained consistently in our patients, with no secondary fractures, although the construct failed in one patient.

LEVEL OF EVIDENCE

Level IV.

Stemmed TKA in a femur with a total hip arthroplasty: is there a safe distance between the stem tips?

When a stemmed TKA is needed in a femur in which a THA is already present, choosing an appropriate length for the TKA stem is crucial. Many surgeons intuitively fear that the distance between the stem tips correlates with the femur risk for fracture (RF). However, to date, no biomechanical data to support this intuition are available. Therefore, in this study, the RF in such a configuration was determined and compared for several activities, using a finite element modeling technique. During gait and sideways falling no difference in RF among different stem lengths was shown. However, a clear threshold appears during four-point bending. Stem tip distances shorter than 110 mm dramatically increased RF and, in osteoporotic bone, will certainly lead to fracture (RF>1) and thus should be avoided.

Cement augmentation of the proximal femoral nail antirotation for the treatment of osteoporotic pertrochanteric fractures--a biomechanical cadaver study

INTRODUCTION

Proximal femoral fractures will gain increasing importance in the future due to the epidemiological development. Osteoporosis is often a limiting factor in the achievement of implant stability. New nailing systems offer the possibility of augmentation of the femoral neck component with cement. The aim of this study was to perform a biomechanical comparison of implant stability in osteoporotic pertrochanteric fractures using the proximal femoral nail antirotation (PFNA, Synthes GmbH, Umkirch, Germany) with cement augmented and non-augmented blades.

MATERIALS AND METHODS

Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DEXA) in six pairs of fresh-frozen human femurs. Standardised pertrochanteric fractures (AO31-A2.3) were treated with a PFNA. Cement augmentation was performed in six constructs. Axial loading was applied according to a single-leg-stance model using a hydraulic testing machine increasing to 1400N over 10,000 cycles. Biomechanical comparisons between the two groups that were comparable concerning BMD, tip-apex-distance and native stiffness were made with regard to postoperative stiffness, survived cycles, load to failure, failure mechanism and axial displacement.

RESULTS

The stiffness of all stabilised femurs was significantly lower than for native specimens (native 702.5±159.6N/mm vs. postoperative 275.4±53.8N/mm, p<0.001). Stiffness after instrumentation was significantly greater for the cement augmented group than for the non-augmented group (300.6±46.7N/mm vs. 250.3±51.6N/mm, respectively, p=0.001). Five of the twelve constructs survived cyclic testing. Statistically significant differences of the BMD were detected between survived and failed constructs (0.79±0.17g/cm(2) vs. 0.45±0.12g/cm(2), respectively, p=0.028). The failure loads for specimens surviving 10,000 cycles were 4611.9±2078.9N in the cement augmented group (n=3) and 4516.3N and 3253.5N in the non-augmented group (n=2). Postoperative stiffness was found to be a positive predictor of maximum force to failure (R(2)=0.83, p=0.02).

CONCLUSIONS

The results of this biomechanical study show that cement augmentation of the PFNA increases the implant stability in osteoporotic pertrochanteric fractures. Further studies are necessary to evaluate this procedure in providing long term clinical results.

What is the risk of stress risers for interprosthetic fractures of the femur? A biomechanical analysis

PURPOSE

Due to increasing life expectancy we see a rising number of joint replacements. Along with the proximal prosthesis in the femur, more and more people have a second implant on the distal ipsilateral side. This might be a retrograde nail or a locking plate to treat distal femur fractures or a constrained knee prosthesis in the case of severe arthrosis. All these constructs can lead to fractures between the implants. The goal of this study was to evaluate the risk of stress risers for interprosthetic fractures of the femur.

METHODS

Thirty human cadaveric femurs were divided into five groups: (1) femurs with a prosthesis on the proximal side only, (2) hip prosthesis on the proximal end and a distal femur nail, (3) femurs with both a hip prosthesis and a constrained knee prosthesis, (4) femurs with a hip prosthesis on the proximal side and a 4.5-mm distal femur locking plate; the locking plate was 230 mm in length, with ten holes in the shaft, and (5) femurs with a proximal hip prosthesis and a 4.5-mm distal femur locking plate; the locking plate was 342 mm in length, with 16 holes in the shaft.

RESULTS

Femurs with a hip prosthesis and knee prosthesis showed significantly higher required fracture force compared to femurs with a hip prosthesis and a distal retrograde nail. Femurs with a distal locking plate of either length showed a higher required fracture force than those with the retrograde nail.

CONCLUSIONS

The highest risk for a fracture in the femur with an existing hip prosthesis comes with a retrograde nail. A distal locking plate for the treatment of supracondylar fractures leads to a higher required fracture force. The implantation of a constrained knee prosthesis that is not loosened on the ipsilateral side does not increase the risk for a fracture.