In-vivo stiffness assessment of distal femur fracture locked plating constructs

doi:10.1016/j.clinbiomech.2018.05.012

Clinical Biomechanics

Volume 56, July 2018, Pages 46-51

https://doi.org/10.1016/j.clinbiomech.2018.05.012 Get rights and content

Highlights

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A custom device was used to measure stiffness of distal femur fixation constructs.
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This study provides estimates for in-vivo plate stiffness for fracture constructs.
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Nonunions remain a prevalent problem; this methodology warrants further study.

Abstract

Background

The purpose of this study was to design and validate a novel stiffness-measuring device using locked plating of distal femur fractures as a model.

Methods

All patients underwent a laterally-based approach, with a bridging locked construct after indirect reduction. A custom and calibrated intraoperative stiffness device was applied and the stiffness of the construct was blindly recorded. Fourteen of twenty-seven patients enrolled with distal femur fractures (AO/OTA 33A and 33C) completed the study. Correlations between stiffness and callus formation, working length, working length/plate length ratio, number of distal locking screws, and fracture pattern were explored.

Findings

Callus and modified radiographic union scale in tibias scores as a linear function of stiffness did not correlate (R2 = 0.06 and 0.07, respectively). Construct working length and working length to plate length ratio did not correlate to stiffness (R2 = 0.18 and 0.16 respectively). A combined delayed and nonunion rate was 14%. Lower extremity measure scores were not statistically different when comparing delayed and nonunion with healed fractures.

Interpretation

The lack of correlation may have been due to the mechanical properties of the plate itself and its large contribution to the overall stiffness of the construct. To our knowledge, clinically relevant stiffness has not been described and this study may provide some estimates. This methodology and these preliminary findings may lay the groundwork for further investigations into this prevalent clinical problem. Other parameters not investigated may play a key role such as body mass index and bone mineral density.

Level of evidence

Diagnostic/Prognostic Level II.

Introduction

Delayed healing or nonunion occurs in an estimated 5–10% of all fractures. Animal and computational studies have demonstrated that the ideal fixation stiffness occurs within a range to allow fracture healing (Fouz et al., 1997; Epari et al., 2007; Ganesh et al., 2005). Excessive stiffness of bone-plate constructs may be responsible for delayed union or nonunion in the treatment of comminuted distal femur fractures (Button et al., 2004; Gardner et al., 2006). Biomechanical studies have shown that plate-screw construct variables can affect stiffness, such as the plate working length, plate length, screw length and number, and using dynamic locked plating (Kowalski et al., 1996; Stoffel et al., 2003; Gardner et al., 2009; Bottlang et al., 2009). Currently, there is no way to objectively assess the stiffness of constructs applied intraoperatively. The exact appropriate stiffness range is unknown. To address this issue, a novel device to intraoperatively quantify the stiffness of locked plating of distal femur fractures was designed and validated. In-vivo stiffness has not previously been quantified intraoperatively. The rationale for this study was to examine and quantify variables of a bone-plate construct for the treatment of distal femur fractures that correlate to the measured stiffness and potentially callus formation. This would be clinically relevant as surgeons could modify the construct variables at the time of application to maximize union rates. The purpose of this study was to design and utilize a novel stiffness-measuring device in the treatment of distal femur fractures fixed with locked plating. To our knowledge at the time of this manuscript no device existed. We hypothesized that a positive correlation would exist between stiffness and callus formation, working length (WL), working length plate length ratio (WL/PL), and the number of distal locking screws.

Section snippets

Design and validation of custom stiffness measurement device

We designed, built, and validated a custom device (Fig. 1, Fig. 2, Fig. 3) to measure stiffness of a distal femoral fracture plate (LISS, Less Invasive Stabilization System, Synthes). The device consists of a screw- driven linear actuator that is manually controlled, in series with a force transducer (75 N, Omega) connected to a data acquisition computer. At each end of the device are two ball joint rod ends that attach to posts or drill guides, which attach at their other ends to the fracture

Results

Construct stiffness did not strongly correlate with either callus score or mRUST score (R² = 0.06 and 0.07, respectively). There was no strong correlation between stiffness and WL or stiffness and WL/PL ratio (R² = 0.18 and 0.16 respectively). Sub analysis of the A and C fracture pattern did show improvement in linear correlation with WL versus stiffness and WL/PL versus stiffness for C-types (R² = 0.59 and 0.32 respectively) (Fig. 6) but not for A-type (both R² = 0.12). When stratified for the

Discussion and conclusions

Delayed unions and nonunions are still an important concern for surgeons treating all fractures. Recently, distal femur fractures have been studied frequently for this complication. Nonunion rates of 0 to 20% for distal femur treated with lateral locking plates have been reported (Henderson et al., 2011a; Henderson et al., 2011b; Ricci et al., 2014; Rodriguez et al., 2014; Rodriguez et al., 2016; Southeast Fracture Consortium, 2016). Stiffness has been implicated for these relatively high

Conflict of interest statement

The following authors have no financial disclosures to report:

Christopher Parks, MD

Amanda Spraggs-Hughes, MA

Matthew Silva, PhD

The following authors have financial disclosures to report:

Christopher M. McAndrew, MD

Dr. McAndrew receives research design consulting fees from Zimmer. He has received payment and travel for speaking from AO North America and AO Trauma, as well as tuition and institutional research support from the National Institutes of Health.

William M. Ricci, MD

Dr. Ricci receives

Source of funding

This research was conducted with support from the Investigator-Initiated Study Program of DePuy Synthes.

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Predictive factors of distal femoral fracture nonunion after lateral locked plating: a retrospective multicenter case-control study of 283 fractures
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Far cortical locking can reduce stiffness of locked plating constructs while retaining construct strength
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Dynamic fixation of distal femur fractures using far cortical locking screws: a prospective observational study
J. Orthop. Trauma
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G. Button et al.
Failure of less invasive stabilization system plates in the distal femur: a report of four cases
J. Orthop. Trauma
(2004)
D.R. Epari et al.
Timely fracture-healing requires optimization of axial fixation stability
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(2007)
A. Fouz et al.
Improved fracture healing with less rigid plates. A biomechanical study in dogs
Clin. Orthop. Relat. Res.
(1997)
V.K. Ganesh et al.
Biomechanics of bone-fracture fixation by stiffness-graded plates in comparison with stainless-steel plates
Biomed. Eng. Online
(2005)
M.J. Gardner et al.
Hybrid locked plating of osteoporotic fractures of the humerus
J. Bone Joint Surg. Am.
(2006)
M.J. Gardner et al.
Stiffness modulation of locking plate constructs using near cortical slotted holes: a preliminary study
J. Orthop. Trauma
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C.E. Henderson et al.
2010 Mid-America Orthopaedic Association physician in training aware: healing complications are common after locked plating for distal femur fractures
Clin. Orthop. Relat. Res.
(2011)

There are more references available in the full text version of this article.

Cited by (10)

An analytical model of lateral condylar plate working length
2023, Clinical Biomechanics
The locking plate is a common device to treat distal femur fractures. Healing is affected by construct stiffness, thus many surgeon-controlled variables such as working length have been examined for their effects on strain at the fracture. No convenient analytical model which aids surgeons in determining working length has yet been described. We propose an analytical model and compare it to finite element analysis and cadaveric biomechanical testing.
First, an analytical model based on a cantilever beam equation was derived. Next, a finite element model was developed based on a CT scan of a “fresh-frozen” cadaveric femur. Third, biomechanical testing in single-leg stance loading was performed on the cadaver. In all methods, strain at the fracture was recorded. An ANCOVA test was conducted to compare the strains.
In all models, as the working length increased so did strain. For strain at the fracture, the shortest working length (35 mm) had a strain of 8% in the analytical model, 9% in the finite element model, and 7% for the cadaver. The longest working length (140 mm) demonstrated strain of 15% in the analytical model, and the finite element and biomechanical tests both demonstrated strain of 14%.
The strain predicted by the analytical model was consistent with the strain observed in both the finite element and biomechanical models. As demonstrated in existing literature, increasing the working length increases strain at the fracture site. Additional work is required to refine and establish validity and reliability of the analytical model.
Biomechanical optimization of the far cortical locking technique for early healing of distal femur fractures
2021, Medical Engineering and Physics
Citation Excerpt :
The current biomechanical effects of plate working length WL on distal femur plating using FCL technology can be compared to prior findings. First, previous distal femurs plated only with standard locking screws showed that AIMFC and AIMNC rose as WL increased [30,31,38,53]; this agrees with present AIMFC results, indicating that FCL screw placement further away from the fracture creates a less rigid construct (Fig. 4). Second, prior distal femurs plated only with standard locking screws showed that AIMFC and AIMNC were not significantly affected when the same number of screws was variously distributed to create the same WL [31]; this confirms current data which had a low 1.5–1.6% coefficient of variation for different FCL screw distribution at a given WL (Fig. 4), suggesting it is mainly the final WL that is of biomechanical importance.
This finite element study optimized far cortical locking (FCL) technology for early callus formation in distal femur fracture fixation with a 9-hole plate using FCL screws proximal to, and standard locking screws distal to, the fracture. Analyses were done for 120 possible FCL screw configurations by varying FCL screw distribution and number. A hip joint force of 700 N (i.e. 100% x body weight) was used, which corresponds to a typical 140 N “toe-touch” foot-to-ground force (i.e. 20% x body weight) suggested to patients immediately after surgery. Increased FCL screw distribution (i.e. shorter plate working length) caused a decrease at the medial side and an increase at the lateral side of the axial interfragmentary motion (AIM), mildly affected shaft and condylar cortex Von Mises max stress (σ_MAX), increased plate σ_MAX, and decreased shaft FCL screw and condylar locking screw σ_MAX. Increased FCL screw number decreased AIM and σ_MAX on the shaft cortex, condylar cortex, plate, and FCL screws, but not condylar screws. The optimal FCL screw configuration had 3 FCL screws in plate holes #1, 5, and 6 (proximal to distal) for optimal AIM of 0.2 - 1 mm and reduce shear fracture motion, thereby encouraging early callus formation.
Biomechanical design using in-vitro finite element modeling of distal femur fracture plates made from semi-rigid materials versus traditional metals for post-operative toe-touch weight-bearing
2021, Medical Engineering and Physics
Citation Excerpt :
A future study could assess changing WL between metal screws just proximal and distal to the fracture [23,27-31,72,91,92] to clearly understand the effect on AIM, plate stresses, and screw stresses. Prior studies showed: (a) increasing WL causes a rise in AIM [27,28,30,91]; (b) a different number or distribution of screws to make the same WL has little influence on AIM [28]; (c) increasing WL causes a drop in plate and screw stresses [72]; (d) increasing WL may or may not improve fracture union rate [29,92]. Lastly, the effect of boundary conditions during future FE models of distal femurs should be considered carefully to ensure physiologically realistic results [93-96].
This proof-of-concept study designs distal femur fracture plates from semi-rigid materials vs. traditional metals for toe-touch weight-bearing recommended to patients immediately after surgery. The two-fold goal was to (a) reduce stress shielding (SS) by increasing cortical bone stress thereby reducing the risk of bone absorption and plate loosening, and (b) reduce delayed healing (DH) via early callus formation by optimizing axial interfragmentary motion (AIM). Finite element analysis was used to design semi-rigid plates whose elastic moduli E ensured plates permitted AIM of 0.2 - 1 mm for early callus formation. A low hip joint force of 700 N (i.e. 100% x body weight) was applied, which corresponds to a typical 140 N toe-touch foot-to-ground force (i.e. 20% x body weight) recommended to patients after surgery. Analysis was done using 2 screw materials (steel or titanium) and types (locked or non-locked). Steel and titanium plates were also analyzed. Semi-rigid plates (vs. metal plates) had lower overall femur/plate construct stiffnesses of 508 - 1482 N/mm, higher cortical bone stresses under the plate by 2.02x - 3.27x thereby reducing SS, and lower E values of 414 - 2302 MPa to permit AIM of 0.2 - 1 mm thereby reducing DH.
How do pilon fractures heal? An analysis of dual plating and bridging callus formation
2020, Injury
Citation Excerpt :
Second, it may be that load to failure at the time of initial fixation, as measured by Busel et al, is not correlated with post-operative callus formation or mRUST scores, and reoperation may not be a sensitive enough outcome to detect more subtle mechanical problems that may impact the patient but not require a second surgery [5]. Screw density and its role in modulation of construct stiffness has been discussed previously in the literature as a factor affecting fracture healing, primarily for distal femur fractures treated with bridge plate constructs [15–18]. One study found that patients with more total screws and higher plate-screw density at the fracture site were at higher risk for failure [15].
1) To determine the effect of single versus dual plate metaphyseal fixation for pilon fractures on callus formation and reoperation rates, 2) to determine the effect of biomechanically matched versus unmatched fixation, and 3) to determine whether patient or surgical factors were independent predictors of bridging callus formation or need for reoperation.
Retrospective comparative study.
Single level one trauma center.
Fifty patients with AO/OTA type C2 or C3 pilon fractures treated with plate fixation.
Internal fixation with a plate and screw construct, with comparisons made between patients with single versus dual plate fixation, and patients treated with biomechanically matched or unmatched fixation.
Modified RUST (mRUST) scores at three and six months and reoperation rate.
At six months, mean mRUST scores were significantly lower in patients treated with dual metaphyseal plates compared to a single plate (8.7 vs 10.4, p=0.046) There were 15 open fractures; eight were treated with supplemental fixation, while seven were treated with single-column fixation. Open fracture (OR 51.05, p=0.008) was a risk factor for reoperation. Screw density between 0.4 and 0.5 was a protective factor against reoperation (OR 0.03, p=0.026). Biomechanically unmatched fixation did not affect mRUST scores or reoperation rates.
Pilon fractures treated with a single plate had more callus formation six months after surgery compared to those treated with dual plate fixation, and there was no difference in reoperation rates. Screw density between 0.4-0.5 was protective against reoperation. These data may serve as the basis of future work to determine the ideal fixation construct for the frequently comminuted metaphysis in pilon fractures. Further work is necessary to determine whether callus formation in these injuries is desirable.
Three
Do working length and proximal screw density influence the velocity of callus formation in distal tibia fractures treated with a medial bridge plate?
2024, European Journal of Orthopaedic Surgery and Traumatology
In vitro analysis and in vivo assessment of fracture complications associated with use of locking plate constructs for stabilization of caprine tibial segmental defects
2023, Journal of Experimental Orthopaedics

View all citing articles on Scopus

^☆: Presented in part at the Annual Meeting of the Orthopaedic Trauma Association, National Harbor, MD, October 2016.

View full text

In-vivo stiffness assessment of distal femur fracture locked plating constructs☆

Highlights

Abstract

Background

Methods

Findings

Interpretation

Level of evidence

Introduction

Section snippets

Design and validation of custom stiffness measurement device

Results

Discussion and conclusions

Conflict of interest statement

Source of funding

Injury

Far cortical locking can reduce stiffness of locked plating constructs while retaining construct strength

J. Bone Joint Surg. Am.

Dynamic fixation of distal femur fractures using far cortical locking screws: a prospective observational study

J. Orthop. Trauma

Failure of less invasive stabilization system plates in the distal femur: a report of four cases

J. Orthop. Trauma

Timely fracture-healing requires optimization of axial fixation stability

J. Bone Joint Surg. Am.

Improved fracture healing with less rigid plates. A biomechanical study in dogs

Clin. Orthop. Relat. Res.

Biomechanics of bone-fracture fixation by stiffness-graded plates in comparison with stainless-steel plates

Biomed. Eng. Online

Hybrid locked plating of osteoporotic fractures of the humerus

J. Bone Joint Surg. Am.

Stiffness modulation of locking plate constructs using near cortical slotted holes: a preliminary study

J. Orthop. Trauma

2010 Mid-America Orthopaedic Association physician in training aware: healing complications are common after locked plating for distal femur fractures

Clin. Orthop. Relat. Res.