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Trochanteric fracture-implant motion during healing – A radiostereometry (RSA) study
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Injury journal homepage: www.elsevier.com/locate/injury
Trochanteric fracture-implant motion during healing – A radiostereometry (RSA) study Alicja J. Bojan* , Anders Jönsson, Hans Granhed, Carl Ekholm, Johan Kärrholm Department of Orthopaedics, Institute of Clinical Sciences at Sahlgrenska Academy, Gothenburg University, Sweden
A R T I C L E I N F O
Article history: Received 28 September 2017 Received in revised form 13 December 2017 Accepted 6 January 2018 Keywords: Cut-out Trochanteric fractures RSA Three-dimensional motions
A B S T R A C T
Cut-out complication remains a major unsolved problem in the treatment of trochanteric hip fractures. A better understanding of the three-dimensional fracture-implant motions is needed to enable further development of clinical strategies and countermeasures. The aim of this clinical study was to characterise and quantify three-dimensional motions between the implant and the bone and between the lag screw and nail of the Gamma nail. Radiostereometry Analysis (RSA) analysis was applied in 20 patients with trochanteric hip fractures treated with an intramedullary nail. The following three-dimensional motions were measured postoperatively, at 1 week, 3, 6 and 12 months: translations of the tip of the lag screw in the femoral head, motions of the lag screw in the nail, femoral head motions relative to the nail and nail movements in the femoral shaft. Cranial migration of the tip of the lag screw dominated over the other two translation components in the femoral head. In all fractures the lag screw slid laterally in the nail and the femoral head moved both laterally and inferiorly towards the nail. All femoral heads translated posteriorly relative to the nail, and rotations occurred in both directions with median values close to zero. The nail tended to retrovert in the femoral shaft. Adverse fracture-implant motions were detected in stable trochanteric hip fractures treated with intramedullary nails with high resolution. Therefore, RSA method can be used to evaluate new implant designs and clinical strategies, which aim to reduce cut-out complications. Future RSA studies should aim at more unstable fractures as these are more likely to fail with cut-out. © 2018 Elsevier Ltd. All rights reserved.
Background The modern operative treatment of trochanteric hip fractures was conceived in the late 19300 s [1]. Since then numerous extramedullary and more recently intramedullary implants have been designed with the aim of improving outcomes. Despite the many incremental improvements, cephalic screw or blade cut-out remains a relatively common complication necessitating revision surgery in the most fragile of orthopaedic patients. The risk factors for the cut-out complication i.e. the perforation of the implant (a screw, pin or blade) through the femoral head, have been identified [2,3]. The consensus is that both reduction of the fracture and postioning of the implant play critical roles in this event, and that the implant selection seems to be less important [4].
* Corresponding author at: Department of Orthopaedics, Sahlgrenska University Hospital/Mölndal, SE-431 80 Mölndal, Sweden. E-mail address: [email protected] (A.J. Bojan).
The cut-out complication remains an unsolved problem and its absolute numbers are likely to increase as the elderly population increases in numbers [5]. Nowadays, it is widely accepted that the nature of the cut-out event is multidirectional with significant rotational and translational displacements. These motions are a direct consequence of the large and dynamic three-dimensional hip forces and moments that act on the proximal femur even in ordinary walking [6]. The multidirectional character was observed by a number of authors in both biomechanical and clinical studies [7–10]. However laboratory testing and retrospective studies on plain radiographs have obvious methodological limitations. Therefore there is still a need for further clinical research into the cut-out mechanism and to understand the interplay between the three dimensional movements of the bone-implant system in patients during the period between surgery and final healing of the fracture. Accordingly, the authors considered the application of Radiosterometric Analysis (RSA) that could enable accurate and precise delineation of the screw and nail implant motions relative to the femoral head. The interplay between head fragment, screw
https://doi.org/10.1016/j.injury.2018.01.005 0020-1383/© 2018 Elsevier Ltd. All rights reserved.
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and nail motions during healing will give us the first detailed insights into how the screw moves within cancellous bone after it’s initial placement. In a previous baseline experimental study [11], we evaluated the accuracy and precision of RSA in a trochanteric hip fracture model fixed with an intramedullary nail. This study demonstrated that the three-dimensional bone and implant motions in trochanteric fractures can be obtained with both high accuracy and precision. This sets the scene for the present study in which the three-dimensional relative motions of the head of the femur to the screw and the screw to the nail are measured in a patient series to gain insights into the nature of implant stability during the healing period. The aim of the present clinical study was to characterise and quantify the three-dimensional motions of the tip of the screw relative to the femoral head, as well as motions of the screw relative to the nail. It is hoped that this study will be a step toward RSA becoming a standard method for testing of new devices for trochanteric fracture fixation where implant migration is used as a predictor for failure [12]. Approval by the local ethics committee was obtained on the 1st June 2009, registration number 100-09. Methods The prospective cohort study was performed at the Sahlgrenska University Hospital in Mölndal, Sweden. 30 patients (median age 82.5 years, 22 females) with pertrochanteric fractures (AO/ASIF 31 A2.1) considered as stable were enrolled in the study. The reason for only including stable fractures was to adapt the RSA method in trochanteric fractures treated with intramedullary implants and to study a uniform group as far as fracture type is concerned. Further inclusion criteria were: a signed informed consent prior to the surgery, ability to walk with or without ambulatory aids and intact mental status. Subsequently, patients were treated with short Gamma nails (Stryker GmbH, Schönkirchen, Germany). All operations were performed under spinal anaesthesia by five orthopaedic surgeons trained in marking the fracture fragments with tantalum beads. The operative procedures were performed on a traction table, guided with the use of the image intensifier. After fracture reduction and insertion of the Gamma nail, the lag screw guide wire was placed in the femoral head aiming for the central– central position as seen on both the antero-posterior and lateral views. Once the lag screw canal was drilled, all the instruments and the nail were removed from the femur while ensuring that the fracture reduction was not lost. Using a specially designed insertion device (UmRSA Biomedical, Umeå, Sweden), six to nine tantalum beads, 1 mm in diameter, were pressed into the trabecular bone of the femoral head though the prepared lag screw canal. In eight patients, three to six beads were placed in the femoral shaft, lateral to the fracture line, through the opening in the lateral cortex and through the opening at the top of the greater trochanter. The nail was then reinserted and the lag screw placed in the femoral head within 1 cm from the subchondral bone and locked dynamically by the set-screw, subsequently, the nail was distally locked. The nail and the lag screw were previously marked by the manufacturer with four tantalum beads each (Fig. 1). All patients underwent their first RSA examination within 24 h postoperatively, prior to any weight bearing or sitting up in bed. After this first assessement, the patients were allowed unrestricted full weight bearing. The subsequent four follow-up RSA examinations were scheduled at 1 week, 3, 6 and 12 months postoperatively. All RSA examinations were performed with the patient in the suppine position, and all the assessements were done by the same x-ray technician using the same protocoll at each time point as follows: The radiostereometric examinations were performed with use of two simultaneously exposing (within 0.3 s) roentgen tubes
Fig. 1. Lag screw marked with 2 tantalum beads at its proximal end.
angulated at about 40 to each other. All examinations were done using an Adora radiographic system (NRT-Nordisk Røntgen Teknik A/S, Hasselager, Denmark) and exposed at 133 kV and 5 mAs, with the use of an uniplanar RSA calibration cage (cage 77, UmRSA Biomedical, Umeå, Sweden). Digital screens (Canon CXDI-50RF, 5.9 pixels, 4096 grayscales, 12-bit) were placed underneath the hip. The three-dimensional position of each marker and the relative movement of the marked fracture and implant components were calculated using specially designed software (UmRSA Biomedical, Umeå, Sweden). In summary, the RSA method enables the accurate and precise determination of position of all RSA the marker beads relative to one another, to be derived from the stereo pairs of X-ray pictures obtained at each time point. The condition number calculated at each RSA examination is a measure of marker distribution which influences the resolution of the measurements. High condition numbers indicate poor marker distribution, while low condition numbers indicate adequte marker distribution. In this study, the accepted median condition number for markers in the femoral head were 50 (range 29–84, n = 20) and for the femoral shaft 78 (range 44–151, n = 8). The corresponding condition numbers for the intramedullary nail (n = 19) and lag screw (n = 20) were 116 (115–120) and 182, range (176–209), respectively. In patient number 4 one bead in the distal tip of the nail was not visible at 6 months follow-up and the condition number increased to 221, thus, this patient was excluded from the evaluations of movements including the nail. The accepted mean error of rigid body fitting was less than 0.35 mm. The following movements were analysed (Fig. 2): 1. Translations of the proximal tip of the lag screw in the femoral head. They were analysed as the average translations in the femoral head of the two tantalum markers placed in the proximal tip of the lag screw (point motion). The analysis was made along the anatomical axes.
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Fig. 2. Movements analysed: 1. Tip of the lag screw translation in the femoral head, 2. Lag screw rotations and translations in the nail, x1 translation (only along x-axis) and x1-, y1-, z1-rotations of the lag screw calculated in the rotated coordinate system 3. Femoral head translations and rotations relative to the nail, x2- translation and rotation of the femoral head calculated along lag screw axis. Movements in relation to y- and z-axis are not accounted for, 4. Nail translations and rotations in the femoral shaft.
2. Translations of the lag screw in the nail along the anatomical axes of the body. Additional calculations of translation (x1, Fig. 2) were made along the transverse axis with the coordinate system rotated to align the transverse axis (x-axis) with the axis of the lag screw [11]. All rotations were measured around the anatomical and the rotated coordinate system in order to present the effect of coordinates rotation on measured motions. Motions were measured at the gravitational centre of the figure created by the four markers in the screw relative to the four markers in the nail (segment motion). 3. Translations and rotations of the femoral head relative to the Gamma nail along the anatomical axes as segment motions and an additional measurement (x2, Fig. 2) along/around the transverse axis (x-axis) in the rotated coordinate system. 4. Translations and rotations of the intramedullary nail in the femoral shaft as segment motion (data available for 8 patients). Two patients died postoperatively, one because of multiorgan failure at day 10, the second one due to unrelated causes 7 months after surgery. One patient had to be excluded because no postoperative RSA examination was performed due to poor medical status. Further seven patients withdrew their consent due to sickness or senile fatigue and did not complete all follow-up examinations. In all, 20 patients could be followed according to the study protocol. 16 patients were female, the median age was 82 years (range: 65–87), the median ASA score was 2 (range 1–3). The mental status of the patients was evaluated with the Pfeiffer score and had to be higher than 3 for inclusion (median value 10, range 3–10). Nine of the fractures occurred on the right side. DXA examination (Dicovery QDR, Hologic Inc.) of the contralateral hip was performed postoperatively during the hospital stay. The median T-score for the group was 2.1 (range 0.9 to 2.9, data available for 17 patients).
Statistics Median and range for all motions are presented. The precision of the RSA measurements was evaluated with use of repeated examinations of the same patient with an interval of about 10 min. The error is presented as 99% confidence interval based on a data scatter around zero (Table 1). Results There were no reoperations during the study period and no postoperative wound infections. 19 fractures were clinically healed at 6 months after surgery. Clinical fracture healing was defined as absence of pain at the fracture site on weight-bearing. The patient presenting with pain on weight-bearing at 6 months postoperatively eventually healed at 12 months. In this case, postoperative radiographs analysis revealed that the set-screw had not been properly engaged into the lag screw. Tip of the lag screw movements in the femoral head Translations above the detection limits in individual cases along any of the axes occurred in 13 patients with the maximum amplitude between 1 week and 3 months (Fig. 3, Table 2). At the final follow-up the proximal tip of the lag screw had migrated in the proximal or distal direction 0.13 mm (range 0.91 to 3.20 mm) (Fig. 4), whereas the medial/lateral and anterior/posterior displacements were less pronounced (Table 3). The two outliers (migration above 2 mm) in Fig. 3 had T-score of 2.1 and 2.9, respectively. Lag screw movements in the nail The lag screw slid laterally () in the nail in all 20 fractures (median 5.00, range 19.01 to 1.90 mm at 12 months). In most
Table 1 Precision: movement detection level for a single observation at 99% confidence interval; x1- translation and rotation of the lag screw calculated along/around its own axis, x2translation and rotation of the femoral head calculated along/around lag screw axis; n.a. not applicable. Motions
1. The tip of the lag screw in the femoral head n = 20 x
Translation (mm) 0.13 Rotation ( ) n.a.
2. The lag screw in the nail n = 19 3. The femoral head to the nail n = 19
4. The nail in the shaft n=8
y
z
vector
x1
x
y
z
x2
x
y
z
x
y
z
0.12 n.a.
0.32 n.a
0.36 n.a.
0.24 2.73
0.37 3.31
0.70 2.42
0.86 0.73
0.21 0.92
0.58 2.05
1.10 1.99
1.79 0.50
0.15 0.60
0.16 2.21
0.34 2.32
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3
3
2 1
2
0
1
-1
0
1 week
3 months
6 months
0 Fig. 3. Total translation of the tip of the lag screw in the femoral head (mm), dashed line: precision level at 99% confidence interval, n = 20.
cases the nail had reached this position already at 3 months postsurgery (Fig. 5, Table 4). The other movements including rotation were much smaller (Tables 4–6) except for the patient with the unlocked lag screw, where the screw rotated about 26 backwards (counter clockwise, for the right hip) in the nail. In other patients the screws showed out of plane movements (toggling) within the limits of the implant system. According to the manufacturer, the tip of the lag screw can toggle in the nail about 1.5 mm in the proximal/distal and anterior/posterior direction. The lag screw is able to rotate along its own longitudinal axis 1 when locked with the set screw unscrewed one 1/4 of a turn according to the operative technique (it rotates 6 when unscrewed one turn and 12.5 when unlocked 1.5 turn, just before complete unlocking). The effect of the rotation of the coordinate system (the transverse axis coincides with the longitudinal axis of the lag screw) can be appreciated when evaluating the rotation of the lag screw in the femoral nail. The varus/valgus and retro-/anteversion of the lag screw become minimal when these movements are assessed along the lagscrew axis, thus confirming that the actual lag screw rotation effect around its own longitudinal axis (backwards and forwards, Tables 5 and 6). Femoral head movements relative to the nail The femoral head translated in the inferior, lateral and posterior direction in all fractures (Table 7). Rotations were more variable and displayed forward/backward rotation up to 11 around the lag screw axis and up to 5 around the sagittal and longitudinal axis (Table 8). Intramedullary nail movements in the femoral shaft The nail had subsided 5 mm into the femoral shaft at 12 months in one patient where the nail was distally locked in a dynamic mode. Additionally, around 2 mm subsidence occurred in two fractures with the nail locked in a static manner. In these patients, there was a visible gap between the locking screw and the proximal aspect of the distal locking hole on the postoperative radiographs. Five other nails were properly statically locked and did not move significantly in the distal direction (Fig. 6). The other notable movement observed was a tendency of the nail to retrovert
Fig. 4. Tip of the lag screw translation in the femoral head along the anatomical yaxis (+) proximal, () distal migration, dashed line: precision level, n = 2.
(rotation around its longitudinal axis) in the femoral shaft (median 3.1, range 1.55 to 6.43 at 12 months). Discussion The radiostereometric analysis enabled quantification of in vivo motions of the intramedullary nail in stable trochanteric fractures with a high precision for translations between 0.12 and 1.79 mm and slightly inferior precision for rotations 0.50–3.31. We were able to detect characteristic motions of the fracture-implant system that are clinically relevant to the research question. For example, cranial migration of the tip of the lag screw dominated over the other two translation components in the femoral head. In all fractures, the lag screw slid laterally in the nail and this was mirrored as expected by the motions of the femoral head moving both laterally and inferiorly towards the nail. The varus/valgus and forward/backward rotation were quite limited in these relatively stable fractures. Interestingly, all femoral heads translated posteriorly relative to the nail and there was a tendency for the nail to retrovert in the femoral shaft. This most likely reflects the biomechanics of the hip joint forces, which show that there are large internal moments generated, even during walking [6], which tend to displace the femoral head posteriorly. All the translation and rotation measures diminished after 3 months. These findings are in accordance with the recent observations made by van Embden et. al [10] in an RSA study examinig the fracture movements in seven trochanteric hip fractures. However, we could still record detectable movements in some patients between 6 and 12 months after surgery. This could be due to variablity in fracture healing, measurement error, or in case of lag screw tip translation in the femoral head, the possibility that a sclerotic cavity formed around the screw thread and enabled the screw to “toggle”. Although the movements of the tip of the lag screw were small (within 4 mm for the y-axis) they were detectable in the majority of the patients. Even so, there were substantial displacements as a result of the fracture impaction during the healing process. The rotations of the femoral head were relatively small probably due to fracture impaction and interdigitation of the fracture fragments [12]. We suspect that the reason for the delayed clinical healing in one patient was the failure to engage the set screw adequately in
Table 2 Number of patients with detectable fracture-implant movements (translations and rotations) between consecutive follow-ups.
1 2 3 1–3
Tip of the lag screw translation in the femoral head n = 20 Lag screw translation/rotation in the nail n = 19 Femoral head translation/rotation versus nail n = 19 Any motions 1–3
0–1 week
1 week – 3 months
3 months –6 months
6 months –12 months
6 18 18 18
13 19 17 20
4 8 3 11
5 3 1 7
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Table 3 Translation of the tip of the lag screw in the femoral head (mm), cumulative, n = 20. Time
Translation of the tip of the lag screw in the femoral head (mm) Median and range
1 week 3 months 6 months 12 months
0
0
x Medial (+) or lateral ()
y Proximal (+) or distal ()
z Anterior (+) or posterior ()
Total translation Vector
0.03 0.10 to 0.15 0.01 0.91 to 0.46 0.01 1.31 to 0.58 0.03 1.52 to 0.57
0.01 0.21 to 0.22 0.07 1.08 to 2.19 0.06 0.98 to 2.94 0.13 0.91 to 3.20
0.11 0.50 0.08 0.80 0.01 0.67 0.18 0.56
0.18 0.05 0.33 0.09 0.39 0.09 0.37 0.09
1 week
3 months
6 months
-5 -10 -15
Fig. 5. Lag screw sliding in the nail along its own axis, medial (+), lateral (), dashed line: precision level, n = 19.
the lag screw grooves. As a consequence, this allowed an excessive movement of the fracture-implant system leading to uncontrolled lag screw sliding and exaggerated rotations of the lag screw and femoral head. We would like to emphasise the importance of the proper engagement of the set screw in the Gamma nail. In unstable fractures, the failure to lock the lag screw is thought to be a contributing factor in the so called “cut-through” complication in the Gamma nail system where the lag screw can be toggled in through the femoral head torwads the acetabulum [13]. The component motions were typically of the order of a few millimetres and their variance was substantial. They ocurred in all subjects and were detectable with RSA in a homogenous stable fracture group operated on with a standardized operative technique. Despite of this variance, which might be influenced by bone quality, BMI, general health and other patient dependent factors, we could detect a motion pattern of the expected (fracture impaction) and less desirable (lag screw migration) movements in this osteosynthesis system. However, we could not describe a cutout model, as the fractures were stable in the present patient group. Unstable fractures should be studied with this method to describe the typical movements that result in the cut-out complication.
to 0.25 to 0.70 to 0.51 to 0.65
to 0.56 to 2.50 to 3.22 to 3.55
Dictated by the geometry of the intramedullary nail, the condition numbers were high for the lag screw (180) and the nail (120), which influenced the precision of the method, especially for the rotations in the transverse and longitudinal axis. This is a wellknown issue of the RSA method, which is dependent on an adequate marker distribution [14]. This study, the first of its kind to use an intramedullary nail/lag screw marked with tantalum beads, showed that RSA can successfully be used to study bone-implant and intrinsic implant motions in trochanteric fractures treated with intramedullary nailing during the healing period. Titanium implants have an advantage as they do not obscure the tantalum beads either in the bone or implant (like in stainless steel implants) on the RSA examination and this makes the analysis feasible [15]. An alternative to using marked implants is to use the CAD (computer aided design) model-based implants [16]. To what extent this method may become applicable to studies of osteosyntheses of different shapes without a considerable loss of resolution is, at present unknown. The radiostereometric method has potential to study cut-out complication in trochanteric fractures. It can complement time and money consuming and often underpowered studies with failure of the osteosynthesis as outcome measure. The small migrations of an implant might be used as predictive measure for an eventual cut-out event, in the same way as implant migration is used in joint replacement research to predict loosening [17,18]. New implants intended to reduce the likelihood of lag screw migration can be evaluated in small study groups with high measurment precision. Our study cannot address the question of patient numbers necessary to detect clinically important differences. However, based on the observed data scatter, using the unpaired t-test with power set at 80% and statistical significance set at p < 0.05, it was calculated that 18 patients are needed in each group to detect a difference of 1 mm of total translation of the tip of the lag screw in the femoral head. The RSA method is gradually being recognized in trauma research as a high resolution non-invasive in vivo technique using three-dimensional motions as a predictor of implant failure and fracture healing. Additionally, it has the potential to enable
Table 4 Translation of the lag screw in the nail (mm), n = 19. Time
Translation of the lag screw in the nail (mm) Median and range
1 week
1.81 8.24 to 0.31 4.90 17.74 to 1.26 4.95 18.76 to 1.84 5.00 19.01 to 1.90
x
3 months 6 months 12 months
1
Medial (+) or lateral ()
x Medial (+) or lateral ()
y Proximal (+) or distal ()
z Anterior (+) or posterior ()
1.32 6.08 to 0.19 3.12 12.64 to 1.27 3.52 13.4 to 1.34 3.47 13.62 to 1.37
1.25 4.77 to 0.38 2.64 10.9 to 0.34 2.70 11.47 to 0.35 2.57 11.59 to 0.37
0.59 2.89 1.46 6.04 1.43 6.42 1.4 6.48
to 0.37 to 0.22 to 0.13 to 0.01
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Table 5 Rotation of the lag screw in the nail ( ) around anatomical axes of the body, n = 19; * absolute value disregards movement direction. Rotation of the lag screw in the nail ( ) Median (absolute mean value *) and range
Time
1week 3months 6months 12months
X Forward (+) or Backward ()
Y Retroversion (+) or Anteversion ()
Z Valgus (+) or Varus ()
0.28 (1.02) 2.6 to 3.02 0.08 (2.45) 22.29 to 3.59 0.19 (2.48) 21.19 to 4.04 0.22 (2.40) 20.80 to 5.26
0.11 (0.98) 2.74 to 3.00 0.14 (2.44) 14.22 to 4.83 0.35 (2.13) 13.34 to 2.94 0.14 (2.20) 13.41 to 2.94
0.09 (0.01) 1.07 to 0.96 0.41 (0.33) 13.33 to 0.45 0.57 (0.51) 12.35 to 0.65 0.63 (0.55) 12.02 to 1.13
Table 6 Rotation of the lag screw in the nail ( ) around rotated axes coordinate system, where x-axis coincides with the axis of the lag screw, n = 19; * absolute value disregards movement direction. Rotation of the lag screw in the nail ( ) Median (absolute mean value*) and range
Time
1week 3months 6months 12months
X Forward (+) or Backward ()
Y Retroversion (+) or Anteversion ()
Z Valgus (+) or Varus ()
0.43 (1.27) 1.4 to 4.35 0.37 (3.53) 28.22 to 10.64 0.11 (3.41) 26.69 to 10.59 0.29 (3.30) 26.32 to 9.61
0.01 (0.50) 0.63 to 3.00 0.13 (0.78) 0.85 to 4.83 0.04 (0.60) 0.85 to 2.39 0.28 (0.68) 0.76 to 2.56
0.17 (0.29) 0.83 to 0.96 0.42 (0.39) 1.3 to 0.96 0.41 (0.59) 1.28 to 1.22 0.25 (0.58) 1.19 to 1.18
Table 7 Translation of the femoral head relative to the nail (mm), cumulative, n = 19. Time
Translation of the femoral head relative to the nail (mm) Median and range X lag screw axis Medial (+) or lateral ()
1.86 8.28 to 0.23 3months 4.94 17.4 to 1.96 6months 4.94 18.47 to 1.87 12months 5.08 18.75 to 1.89 1week
X anatomical axis Medial (+) or lateral ()
Y anatomical axis Proximal (+) or distal ()
Z anatomical axis Anterior (+) or posterior ()
1.54 5.76 to 0.74 2.81 11.85 to 0.31 3.15 12.70 to 0.94 3.23 13.13 to 0.84
1.09 5.01 to 0.33 3.52 10.26 to 1.37 3.57 10.79 to 1.40 3.58 10.84 to 1.31
0.46 3.21 to 1.49 1.50 7.66 to 0.27 1.40 8.04 to 0.25 1.38 7.94 to 0.07
Table 8 Rotation of the femoral head relative to the nail ( ), cumulative, n = 19. Rotation of the femoral head relative to the nail ( ) Median and range
Time X lag screw axis Forward (+) or Backward () 0.5 4.24 to 6.69 3months 0.66 8.51 to 11.38 6months 0.38 8.96 to 10.51 12months 0.62 9.37 to 10.24
1week
X anatomical axis Forward (+) or Backward ()
Y anatomical axis Retroversion (+) or Anteversion ()
Z anatomical axis Valgus (+) or Varus ()
0.14 2.61 to 5.05 0.81 4.54 to 8.46 0.65 4.42 to 8.49 0.83 4.6 to 9.37
0.97 2.96 0.48 5.57 0.42 5.58 1.10 5.59
0.01 0.96 to 1.8 0.31 3.53 to 4.96 0.39 3.81 to 4.55 0.81 4.17 to 4.22
comparison of different treatment modalities and to guide rehabilitation [15,19]. Conclusions For the first time, the three-dimensional fracture-implant motions of trochanteric hip fractures treated with intramedullary
to 3.95 to 5.77 to 6.19 to 5.52
nails were successfully evaluated in vivo using the RSA method. Therefore, RSA seems to have the potential to be a unique tool in evaluating new implant designs, especially those aiming to decrease the cut-out complication. Future studies should include more unstable fractures in order to characterise their in vivo biomechanics as they are the ones most prone to mechanical complications.
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0
[4]
-1 -2 -3 -4
[5] [6] [7]
-5 [8] Fig. 6. Nail translation in the femoral shaft along the longitudinal nail axis (y), (+) proximal, () distal, n = 8.
Conflict of interest statement The authors, individually and collectively, have no financial, personal or ethical conflicts of interest to report. The study received financial support from IngaBritt och Arne Lundbergs research fund and Swedish Research Council funding for clinical research in medicine and from Stryker GmbH, Schönkirchen, Germany. Funds from Stryker GmbH were solely used to finance the RSA examinations. The funding parties were not involved in the study design, collection, analysis and interpretation of data, the writing of the manuscript or the decision to submit the manuscript for publication.
[9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
References [1] Thornton L. The treatment of trochanteric fractures of the femur: two new methods. Piedmont Hosp Bull 1937;10:21–37. [2] Baumgaertner MR, Curtin SL, Lindskog DM, Keggi JM. The value of the tip-apex distance in predicting failure of fixation of peritrochanteric fractures of the hip. J Bone Joint Surg Am 1995;77:1058–64. [3] Andruszkow H, Frink M, Fromke C, Matityahu A, Zeckey C, Mommsen P, et al. Tip apex distance, hip screw placement, and neck shaft angle as potential risk
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factors for cut-out failure of hip screws after surgical treatment of intertrochanteric fractures. Int Orthop 2012;36:2347–54. Matre K, Vinje T, Havelin LI, Gjertsen JE, Furnes O, Espehaug B, et al. TRIGEN INTERTAN intramedullary nail versus sliding hip screw: a prospective, randomized multicenter study on pain, function, and complications in 684 patients with an intertrochanteric or subtrochanteric fracture and one year of follow-up. J Bone Joint Surg Am Vol 2013;95:200–8. Rosengren BE, Karlsson MK. The annual number of hip fractures in Sweden will double from year 2002 to 2050. Acta Orthop 2014;85:234–7. Bergmann G, Graichen F, Rohlmann A. Hip joint loading during walking and running, measured in two patients. J Biomechnol 1993;26:969–90. Lustenberger A, Bekic J, Ganz R. Rotational instability of trochanteric femoral fractures secured with the dynamic hip screw. A radiologic analysis. Der Unfallchirurg 1995;98:514–7. Ehmke LW, Fitzpatrick DC, Krieg JC, Madey SM, Bottlang M. Lag screws for hip fracture fixation: evaluation of migration resistance under simulated walking. J Orthop Res 2005;23:1329–35. Bojan AJ, Beimel C, Taglang G, Collin D, Ekholm C, Jonsson A. Critical factors in cut-out complication after Gamma Nail treatment of proximal femoral fractures. BMC Musculoskelet Disord 2013;14:1. van Embden D, Stollenwerck GA, Koster LA, Kaptein BL, Nelissen RG, Schipper IB. The stability of fixation of proximal femoral fractures: a radiostereometric analysis. Bone Joint J 2015;97-B:391–7. Bojan AJ, Bragdon C, Jonsson A, Ekholm C, Karrholm J. Three-dimensional bone-implant movements in trochanteric hip fractures: precision and accuracy of radiostereometric analysis in a phantom model. J Orthop Res 2015. Mattsson P, Larsson S. Unstable trochanteric fractures augmented with calcium phosphate cement: a prospective randomized study using radiostereometry to measure fracture stability. Scand J Surg 2004;93:223–8. Weil YA, Gardner MJ, Mikhail G, Pierson G, Helfet DL, Lorich DG. Medial migration of intramedullary hip fixation devices: a biomechanical analysis. Arch Orthop Trauma Surg 2008;128:227–34. Ryd L, Yuan X, Lofgren H. Methods for determining the accuracy of radiostereometric analysis (RSA). Acta Orthop Scand 2000;71:403–8. Madanat R, Strandberg N, Moritz N, Mattila K, Vahlberg T, Aro HT. Radiostereometric analysis in measurements of migration and inducible micromotion in intra-articular distal radius fractures treated with a volar plate. J Orthop Trauma 2012;26:e153–60. Valstar ER, de Jong FW, Vrooman HA, Rozing PM, Reiber JH. Model-based Roentgen stereophotogrammetry of orthopaedic implants. J Biomech 2001;34:715–22. Mjoberg B, Selvik G, Hansson LI, Rosenqvist R, Onnerfalt R. Mechanical loosening of total hip prostheses: a radiographic and roentgen stereophotogrammetric study. J Bone Joint Surg Br Vol 1986;68:770–4. Ryd L. Roentgen stereophotogrammetric analysis of prosthetic fixation in the hip and knee joint. Clin Orthop Relat Res 1992;5:6–65. Augat P, Faschingbauer M, Seide K, Tobita K, Callary SA, Solomon LB, et al. Biomechanical methods for the assessment of fracture repair. Injury 2014;45 (Suppl 2):S32–8.
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Contents lists available at ScienceDirect
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Trochanteric fracture-implant motion during healing – A radiostereometry (RSA) study Alicja J. Bojan* , Anders Jönsson, Hans Granhed, Carl Ekholm, Johan Kärrholm Department of Orthopaedics, Institute of Clinical Sciences at Sahlgrenska Academy, Gothenburg University, Sweden
A R T I C L E I N F O
Article history: Received 28 September 2017 Received in revised form 13 December 2017 Accepted 6 January 2018 Keywords: Cut-out Trochanteric fractures RSA Three-dimensional motions
A B S T R A C T
Cut-out complication remains a major unsolved problem in the treatment of trochanteric hip fractures. A better understanding of the three-dimensional fracture-implant motions is needed to enable further development of clinical strategies and countermeasures. The aim of this clinical study was to characterise and quantify three-dimensional motions between the implant and the bone and between the lag screw and nail of the Gamma nail. Radiostereometry Analysis (RSA) analysis was applied in 20 patients with trochanteric hip fractures treated with an intramedullary nail. The following three-dimensional motions were measured postoperatively, at 1 week, 3, 6 and 12 months: translations of the tip of the lag screw in the femoral head, motions of the lag screw in the nail, femoral head motions relative to the nail and nail movements in the femoral shaft. Cranial migration of the tip of the lag screw dominated over the other two translation components in the femoral head. In all fractures the lag screw slid laterally in the nail and the femoral head moved both laterally and inferiorly towards the nail. All femoral heads translated posteriorly relative to the nail, and rotations occurred in both directions with median values close to zero. The nail tended to retrovert in the femoral shaft. Adverse fracture-implant motions were detected in stable trochanteric hip fractures treated with intramedullary nails with high resolution. Therefore, RSA method can be used to evaluate new implant designs and clinical strategies, which aim to reduce cut-out complications. Future RSA studies should aim at more unstable fractures as these are more likely to fail with cut-out. © 2018 Elsevier Ltd. All rights reserved.
Background The modern operative treatment of trochanteric hip fractures was conceived in the late 19300 s [1]. Since then numerous extramedullary and more recently intramedullary implants have been designed with the aim of improving outcomes. Despite the many incremental improvements, cephalic screw or blade cut-out remains a relatively common complication necessitating revision surgery in the most fragile of orthopaedic patients. The risk factors for the cut-out complication i.e. the perforation of the implant (a screw, pin or blade) through the femoral head, have been identified [2,3]. The consensus is that both reduction of the fracture and postioning of the implant play critical roles in this event, and that the implant selection seems to be less important [4].
* Corresponding author at: Department of Orthopaedics, Sahlgrenska University Hospital/Mölndal, SE-431 80 Mölndal, Sweden. E-mail address: [email protected] (A.J. Bojan).
The cut-out complication remains an unsolved problem and its absolute numbers are likely to increase as the elderly population increases in numbers [5]. Nowadays, it is widely accepted that the nature of the cut-out event is multidirectional with significant rotational and translational displacements. These motions are a direct consequence of the large and dynamic three-dimensional hip forces and moments that act on the proximal femur even in ordinary walking [6]. The multidirectional character was observed by a number of authors in both biomechanical and clinical studies [7–10]. However laboratory testing and retrospective studies on plain radiographs have obvious methodological limitations. Therefore there is still a need for further clinical research into the cut-out mechanism and to understand the interplay between the three dimensional movements of the bone-implant system in patients during the period between surgery and final healing of the fracture. Accordingly, the authors considered the application of Radiosterometric Analysis (RSA) that could enable accurate and precise delineation of the screw and nail implant motions relative to the femoral head. The interplay between head fragment, screw
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and nail motions during healing will give us the first detailed insights into how the screw moves within cancellous bone after it’s initial placement. In a previous baseline experimental study [11], we evaluated the accuracy and precision of RSA in a trochanteric hip fracture model fixed with an intramedullary nail. This study demonstrated that the three-dimensional bone and implant motions in trochanteric fractures can be obtained with both high accuracy and precision. This sets the scene for the present study in which the three-dimensional relative motions of the head of the femur to the screw and the screw to the nail are measured in a patient series to gain insights into the nature of implant stability during the healing period. The aim of the present clinical study was to characterise and quantify the three-dimensional motions of the tip of the screw relative to the femoral head, as well as motions of the screw relative to the nail. It is hoped that this study will be a step toward RSA becoming a standard method for testing of new devices for trochanteric fracture fixation where implant migration is used as a predictor for failure [12]. Approval by the local ethics committee was obtained on the 1st June 2009, registration number 100-09. Methods The prospective cohort study was performed at the Sahlgrenska University Hospital in Mölndal, Sweden. 30 patients (median age 82.5 years, 22 females) with pertrochanteric fractures (AO/ASIF 31 A2.1) considered as stable were enrolled in the study. The reason for only including stable fractures was to adapt the RSA method in trochanteric fractures treated with intramedullary implants and to study a uniform group as far as fracture type is concerned. Further inclusion criteria were: a signed informed consent prior to the surgery, ability to walk with or without ambulatory aids and intact mental status. Subsequently, patients were treated with short Gamma nails (Stryker GmbH, Schönkirchen, Germany). All operations were performed under spinal anaesthesia by five orthopaedic surgeons trained in marking the fracture fragments with tantalum beads. The operative procedures were performed on a traction table, guided with the use of the image intensifier. After fracture reduction and insertion of the Gamma nail, the lag screw guide wire was placed in the femoral head aiming for the central– central position as seen on both the antero-posterior and lateral views. Once the lag screw canal was drilled, all the instruments and the nail were removed from the femur while ensuring that the fracture reduction was not lost. Using a specially designed insertion device (UmRSA Biomedical, Umeå, Sweden), six to nine tantalum beads, 1 mm in diameter, were pressed into the trabecular bone of the femoral head though the prepared lag screw canal. In eight patients, three to six beads were placed in the femoral shaft, lateral to the fracture line, through the opening in the lateral cortex and through the opening at the top of the greater trochanter. The nail was then reinserted and the lag screw placed in the femoral head within 1 cm from the subchondral bone and locked dynamically by the set-screw, subsequently, the nail was distally locked. The nail and the lag screw were previously marked by the manufacturer with four tantalum beads each (Fig. 1). All patients underwent their first RSA examination within 24 h postoperatively, prior to any weight bearing or sitting up in bed. After this first assessement, the patients were allowed unrestricted full weight bearing. The subsequent four follow-up RSA examinations were scheduled at 1 week, 3, 6 and 12 months postoperatively. All RSA examinations were performed with the patient in the suppine position, and all the assessements were done by the same x-ray technician using the same protocoll at each time point as follows: The radiostereometric examinations were performed with use of two simultaneously exposing (within 0.3 s) roentgen tubes
Fig. 1. Lag screw marked with 2 tantalum beads at its proximal end.
angulated at about 40 to each other. All examinations were done using an Adora radiographic system (NRT-Nordisk Røntgen Teknik A/S, Hasselager, Denmark) and exposed at 133 kV and 5 mAs, with the use of an uniplanar RSA calibration cage (cage 77, UmRSA Biomedical, Umeå, Sweden). Digital screens (Canon CXDI-50RF, 5.9 pixels, 4096 grayscales, 12-bit) were placed underneath the hip. The three-dimensional position of each marker and the relative movement of the marked fracture and implant components were calculated using specially designed software (UmRSA Biomedical, Umeå, Sweden). In summary, the RSA method enables the accurate and precise determination of position of all RSA the marker beads relative to one another, to be derived from the stereo pairs of X-ray pictures obtained at each time point. The condition number calculated at each RSA examination is a measure of marker distribution which influences the resolution of the measurements. High condition numbers indicate poor marker distribution, while low condition numbers indicate adequte marker distribution. In this study, the accepted median condition number for markers in the femoral head were 50 (range 29–84, n = 20) and for the femoral shaft 78 (range 44–151, n = 8). The corresponding condition numbers for the intramedullary nail (n = 19) and lag screw (n = 20) were 116 (115–120) and 182, range (176–209), respectively. In patient number 4 one bead in the distal tip of the nail was not visible at 6 months follow-up and the condition number increased to 221, thus, this patient was excluded from the evaluations of movements including the nail. The accepted mean error of rigid body fitting was less than 0.35 mm. The following movements were analysed (Fig. 2): 1. Translations of the proximal tip of the lag screw in the femoral head. They were analysed as the average translations in the femoral head of the two tantalum markers placed in the proximal tip of the lag screw (point motion). The analysis was made along the anatomical axes.
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Fig. 2. Movements analysed: 1. Tip of the lag screw translation in the femoral head, 2. Lag screw rotations and translations in the nail, x1 translation (only along x-axis) and x1-, y1-, z1-rotations of the lag screw calculated in the rotated coordinate system 3. Femoral head translations and rotations relative to the nail, x2- translation and rotation of the femoral head calculated along lag screw axis. Movements in relation to y- and z-axis are not accounted for, 4. Nail translations and rotations in the femoral shaft.
2. Translations of the lag screw in the nail along the anatomical axes of the body. Additional calculations of translation (x1, Fig. 2) were made along the transverse axis with the coordinate system rotated to align the transverse axis (x-axis) with the axis of the lag screw [11]. All rotations were measured around the anatomical and the rotated coordinate system in order to present the effect of coordinates rotation on measured motions. Motions were measured at the gravitational centre of the figure created by the four markers in the screw relative to the four markers in the nail (segment motion). 3. Translations and rotations of the femoral head relative to the Gamma nail along the anatomical axes as segment motions and an additional measurement (x2, Fig. 2) along/around the transverse axis (x-axis) in the rotated coordinate system. 4. Translations and rotations of the intramedullary nail in the femoral shaft as segment motion (data available for 8 patients). Two patients died postoperatively, one because of multiorgan failure at day 10, the second one due to unrelated causes 7 months after surgery. One patient had to be excluded because no postoperative RSA examination was performed due to poor medical status. Further seven patients withdrew their consent due to sickness or senile fatigue and did not complete all follow-up examinations. In all, 20 patients could be followed according to the study protocol. 16 patients were female, the median age was 82 years (range: 65–87), the median ASA score was 2 (range 1–3). The mental status of the patients was evaluated with the Pfeiffer score and had to be higher than 3 for inclusion (median value 10, range 3–10). Nine of the fractures occurred on the right side. DXA examination (Dicovery QDR, Hologic Inc.) of the contralateral hip was performed postoperatively during the hospital stay. The median T-score for the group was 2.1 (range 0.9 to 2.9, data available for 17 patients).
Statistics Median and range for all motions are presented. The precision of the RSA measurements was evaluated with use of repeated examinations of the same patient with an interval of about 10 min. The error is presented as 99% confidence interval based on a data scatter around zero (Table 1). Results There were no reoperations during the study period and no postoperative wound infections. 19 fractures were clinically healed at 6 months after surgery. Clinical fracture healing was defined as absence of pain at the fracture site on weight-bearing. The patient presenting with pain on weight-bearing at 6 months postoperatively eventually healed at 12 months. In this case, postoperative radiographs analysis revealed that the set-screw had not been properly engaged into the lag screw. Tip of the lag screw movements in the femoral head Translations above the detection limits in individual cases along any of the axes occurred in 13 patients with the maximum amplitude between 1 week and 3 months (Fig. 3, Table 2). At the final follow-up the proximal tip of the lag screw had migrated in the proximal or distal direction 0.13 mm (range 0.91 to 3.20 mm) (Fig. 4), whereas the medial/lateral and anterior/posterior displacements were less pronounced (Table 3). The two outliers (migration above 2 mm) in Fig. 3 had T-score of 2.1 and 2.9, respectively. Lag screw movements in the nail The lag screw slid laterally () in the nail in all 20 fractures (median 5.00, range 19.01 to 1.90 mm at 12 months). In most
Table 1 Precision: movement detection level for a single observation at 99% confidence interval; x1- translation and rotation of the lag screw calculated along/around its own axis, x2translation and rotation of the femoral head calculated along/around lag screw axis; n.a. not applicable. Motions
1. The tip of the lag screw in the femoral head n = 20 x
Translation (mm) 0.13 Rotation ( ) n.a.
2. The lag screw in the nail n = 19 3. The femoral head to the nail n = 19
4. The nail in the shaft n=8
y
z
vector
x1
x
y
z
x2
x
y
z
x
y
z
0.12 n.a.
0.32 n.a
0.36 n.a.
0.24 2.73
0.37 3.31
0.70 2.42
0.86 0.73
0.21 0.92
0.58 2.05
1.10 1.99
1.79 0.50
0.15 0.60
0.16 2.21
0.34 2.32
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3
3
2 1
2
0
1
-1
0
1 week
3 months
6 months
0 Fig. 3. Total translation of the tip of the lag screw in the femoral head (mm), dashed line: precision level at 99% confidence interval, n = 20.
cases the nail had reached this position already at 3 months postsurgery (Fig. 5, Table 4). The other movements including rotation were much smaller (Tables 4–6) except for the patient with the unlocked lag screw, where the screw rotated about 26 backwards (counter clockwise, for the right hip) in the nail. In other patients the screws showed out of plane movements (toggling) within the limits of the implant system. According to the manufacturer, the tip of the lag screw can toggle in the nail about 1.5 mm in the proximal/distal and anterior/posterior direction. The lag screw is able to rotate along its own longitudinal axis 1 when locked with the set screw unscrewed one 1/4 of a turn according to the operative technique (it rotates 6 when unscrewed one turn and 12.5 when unlocked 1.5 turn, just before complete unlocking). The effect of the rotation of the coordinate system (the transverse axis coincides with the longitudinal axis of the lag screw) can be appreciated when evaluating the rotation of the lag screw in the femoral nail. The varus/valgus and retro-/anteversion of the lag screw become minimal when these movements are assessed along the lagscrew axis, thus confirming that the actual lag screw rotation effect around its own longitudinal axis (backwards and forwards, Tables 5 and 6). Femoral head movements relative to the nail The femoral head translated in the inferior, lateral and posterior direction in all fractures (Table 7). Rotations were more variable and displayed forward/backward rotation up to 11 around the lag screw axis and up to 5 around the sagittal and longitudinal axis (Table 8). Intramedullary nail movements in the femoral shaft The nail had subsided 5 mm into the femoral shaft at 12 months in one patient where the nail was distally locked in a dynamic mode. Additionally, around 2 mm subsidence occurred in two fractures with the nail locked in a static manner. In these patients, there was a visible gap between the locking screw and the proximal aspect of the distal locking hole on the postoperative radiographs. Five other nails were properly statically locked and did not move significantly in the distal direction (Fig. 6). The other notable movement observed was a tendency of the nail to retrovert
Fig. 4. Tip of the lag screw translation in the femoral head along the anatomical yaxis (+) proximal, () distal migration, dashed line: precision level, n = 2.
(rotation around its longitudinal axis) in the femoral shaft (median 3.1, range 1.55 to 6.43 at 12 months). Discussion The radiostereometric analysis enabled quantification of in vivo motions of the intramedullary nail in stable trochanteric fractures with a high precision for translations between 0.12 and 1.79 mm and slightly inferior precision for rotations 0.50–3.31. We were able to detect characteristic motions of the fracture-implant system that are clinically relevant to the research question. For example, cranial migration of the tip of the lag screw dominated over the other two translation components in the femoral head. In all fractures, the lag screw slid laterally in the nail and this was mirrored as expected by the motions of the femoral head moving both laterally and inferiorly towards the nail. The varus/valgus and forward/backward rotation were quite limited in these relatively stable fractures. Interestingly, all femoral heads translated posteriorly relative to the nail and there was a tendency for the nail to retrovert in the femoral shaft. This most likely reflects the biomechanics of the hip joint forces, which show that there are large internal moments generated, even during walking [6], which tend to displace the femoral head posteriorly. All the translation and rotation measures diminished after 3 months. These findings are in accordance with the recent observations made by van Embden et. al [10] in an RSA study examinig the fracture movements in seven trochanteric hip fractures. However, we could still record detectable movements in some patients between 6 and 12 months after surgery. This could be due to variablity in fracture healing, measurement error, or in case of lag screw tip translation in the femoral head, the possibility that a sclerotic cavity formed around the screw thread and enabled the screw to “toggle”. Although the movements of the tip of the lag screw were small (within 4 mm for the y-axis) they were detectable in the majority of the patients. Even so, there were substantial displacements as a result of the fracture impaction during the healing process. The rotations of the femoral head were relatively small probably due to fracture impaction and interdigitation of the fracture fragments [12]. We suspect that the reason for the delayed clinical healing in one patient was the failure to engage the set screw adequately in
Table 2 Number of patients with detectable fracture-implant movements (translations and rotations) between consecutive follow-ups.
1 2 3 1–3
Tip of the lag screw translation in the femoral head n = 20 Lag screw translation/rotation in the nail n = 19 Femoral head translation/rotation versus nail n = 19 Any motions 1–3
0–1 week
1 week – 3 months
3 months –6 months
6 months –12 months
6 18 18 18
13 19 17 20
4 8 3 11
5 3 1 7
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Table 3 Translation of the tip of the lag screw in the femoral head (mm), cumulative, n = 20. Time
Translation of the tip of the lag screw in the femoral head (mm) Median and range
1 week 3 months 6 months 12 months
0
0
x Medial (+) or lateral ()
y Proximal (+) or distal ()
z Anterior (+) or posterior ()
Total translation Vector
0.03 0.10 to 0.15 0.01 0.91 to 0.46 0.01 1.31 to 0.58 0.03 1.52 to 0.57
0.01 0.21 to 0.22 0.07 1.08 to 2.19 0.06 0.98 to 2.94 0.13 0.91 to 3.20
0.11 0.50 0.08 0.80 0.01 0.67 0.18 0.56
0.18 0.05 0.33 0.09 0.39 0.09 0.37 0.09
1 week
3 months
6 months
-5 -10 -15
Fig. 5. Lag screw sliding in the nail along its own axis, medial (+), lateral (), dashed line: precision level, n = 19.
the lag screw grooves. As a consequence, this allowed an excessive movement of the fracture-implant system leading to uncontrolled lag screw sliding and exaggerated rotations of the lag screw and femoral head. We would like to emphasise the importance of the proper engagement of the set screw in the Gamma nail. In unstable fractures, the failure to lock the lag screw is thought to be a contributing factor in the so called “cut-through” complication in the Gamma nail system where the lag screw can be toggled in through the femoral head torwads the acetabulum [13]. The component motions were typically of the order of a few millimetres and their variance was substantial. They ocurred in all subjects and were detectable with RSA in a homogenous stable fracture group operated on with a standardized operative technique. Despite of this variance, which might be influenced by bone quality, BMI, general health and other patient dependent factors, we could detect a motion pattern of the expected (fracture impaction) and less desirable (lag screw migration) movements in this osteosynthesis system. However, we could not describe a cutout model, as the fractures were stable in the present patient group. Unstable fractures should be studied with this method to describe the typical movements that result in the cut-out complication.
to 0.25 to 0.70 to 0.51 to 0.65
to 0.56 to 2.50 to 3.22 to 3.55
Dictated by the geometry of the intramedullary nail, the condition numbers were high for the lag screw (180) and the nail (120), which influenced the precision of the method, especially for the rotations in the transverse and longitudinal axis. This is a wellknown issue of the RSA method, which is dependent on an adequate marker distribution [14]. This study, the first of its kind to use an intramedullary nail/lag screw marked with tantalum beads, showed that RSA can successfully be used to study bone-implant and intrinsic implant motions in trochanteric fractures treated with intramedullary nailing during the healing period. Titanium implants have an advantage as they do not obscure the tantalum beads either in the bone or implant (like in stainless steel implants) on the RSA examination and this makes the analysis feasible [15]. An alternative to using marked implants is to use the CAD (computer aided design) model-based implants [16]. To what extent this method may become applicable to studies of osteosyntheses of different shapes without a considerable loss of resolution is, at present unknown. The radiostereometric method has potential to study cut-out complication in trochanteric fractures. It can complement time and money consuming and often underpowered studies with failure of the osteosynthesis as outcome measure. The small migrations of an implant might be used as predictive measure for an eventual cut-out event, in the same way as implant migration is used in joint replacement research to predict loosening [17,18]. New implants intended to reduce the likelihood of lag screw migration can be evaluated in small study groups with high measurment precision. Our study cannot address the question of patient numbers necessary to detect clinically important differences. However, based on the observed data scatter, using the unpaired t-test with power set at 80% and statistical significance set at p < 0.05, it was calculated that 18 patients are needed in each group to detect a difference of 1 mm of total translation of the tip of the lag screw in the femoral head. The RSA method is gradually being recognized in trauma research as a high resolution non-invasive in vivo technique using three-dimensional motions as a predictor of implant failure and fracture healing. Additionally, it has the potential to enable
Table 4 Translation of the lag screw in the nail (mm), n = 19. Time
Translation of the lag screw in the nail (mm) Median and range
1 week
1.81 8.24 to 0.31 4.90 17.74 to 1.26 4.95 18.76 to 1.84 5.00 19.01 to 1.90
x
3 months 6 months 12 months
1
Medial (+) or lateral ()
x Medial (+) or lateral ()
y Proximal (+) or distal ()
z Anterior (+) or posterior ()
1.32 6.08 to 0.19 3.12 12.64 to 1.27 3.52 13.4 to 1.34 3.47 13.62 to 1.37
1.25 4.77 to 0.38 2.64 10.9 to 0.34 2.70 11.47 to 0.35 2.57 11.59 to 0.37
0.59 2.89 1.46 6.04 1.43 6.42 1.4 6.48
to 0.37 to 0.22 to 0.13 to 0.01
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Table 5 Rotation of the lag screw in the nail ( ) around anatomical axes of the body, n = 19; * absolute value disregards movement direction. Rotation of the lag screw in the nail ( ) Median (absolute mean value *) and range
Time
1week 3months 6months 12months
X Forward (+) or Backward ()
Y Retroversion (+) or Anteversion ()
Z Valgus (+) or Varus ()
0.28 (1.02) 2.6 to 3.02 0.08 (2.45) 22.29 to 3.59 0.19 (2.48) 21.19 to 4.04 0.22 (2.40) 20.80 to 5.26
0.11 (0.98) 2.74 to 3.00 0.14 (2.44) 14.22 to 4.83 0.35 (2.13) 13.34 to 2.94 0.14 (2.20) 13.41 to 2.94
0.09 (0.01) 1.07 to 0.96 0.41 (0.33) 13.33 to 0.45 0.57 (0.51) 12.35 to 0.65 0.63 (0.55) 12.02 to 1.13
Table 6 Rotation of the lag screw in the nail ( ) around rotated axes coordinate system, where x-axis coincides with the axis of the lag screw, n = 19; * absolute value disregards movement direction. Rotation of the lag screw in the nail ( ) Median (absolute mean value*) and range
Time
1week 3months 6months 12months
X Forward (+) or Backward ()
Y Retroversion (+) or Anteversion ()
Z Valgus (+) or Varus ()
0.43 (1.27) 1.4 to 4.35 0.37 (3.53) 28.22 to 10.64 0.11 (3.41) 26.69 to 10.59 0.29 (3.30) 26.32 to 9.61
0.01 (0.50) 0.63 to 3.00 0.13 (0.78) 0.85 to 4.83 0.04 (0.60) 0.85 to 2.39 0.28 (0.68) 0.76 to 2.56
0.17 (0.29) 0.83 to 0.96 0.42 (0.39) 1.3 to 0.96 0.41 (0.59) 1.28 to 1.22 0.25 (0.58) 1.19 to 1.18
Table 7 Translation of the femoral head relative to the nail (mm), cumulative, n = 19. Time
Translation of the femoral head relative to the nail (mm) Median and range X lag screw axis Medial (+) or lateral ()
1.86 8.28 to 0.23 3months 4.94 17.4 to 1.96 6months 4.94 18.47 to 1.87 12months 5.08 18.75 to 1.89 1week
X anatomical axis Medial (+) or lateral ()
Y anatomical axis Proximal (+) or distal ()
Z anatomical axis Anterior (+) or posterior ()
1.54 5.76 to 0.74 2.81 11.85 to 0.31 3.15 12.70 to 0.94 3.23 13.13 to 0.84
1.09 5.01 to 0.33 3.52 10.26 to 1.37 3.57 10.79 to 1.40 3.58 10.84 to 1.31
0.46 3.21 to 1.49 1.50 7.66 to 0.27 1.40 8.04 to 0.25 1.38 7.94 to 0.07
Table 8 Rotation of the femoral head relative to the nail ( ), cumulative, n = 19. Rotation of the femoral head relative to the nail ( ) Median and range
Time X lag screw axis Forward (+) or Backward () 0.5 4.24 to 6.69 3months 0.66 8.51 to 11.38 6months 0.38 8.96 to 10.51 12months 0.62 9.37 to 10.24
1week
X anatomical axis Forward (+) or Backward ()
Y anatomical axis Retroversion (+) or Anteversion ()
Z anatomical axis Valgus (+) or Varus ()
0.14 2.61 to 5.05 0.81 4.54 to 8.46 0.65 4.42 to 8.49 0.83 4.6 to 9.37
0.97 2.96 0.48 5.57 0.42 5.58 1.10 5.59
0.01 0.96 to 1.8 0.31 3.53 to 4.96 0.39 3.81 to 4.55 0.81 4.17 to 4.22
comparison of different treatment modalities and to guide rehabilitation [15,19]. Conclusions For the first time, the three-dimensional fracture-implant motions of trochanteric hip fractures treated with intramedullary
to 3.95 to 5.77 to 6.19 to 5.52
nails were successfully evaluated in vivo using the RSA method. Therefore, RSA seems to have the potential to be a unique tool in evaluating new implant designs, especially those aiming to decrease the cut-out complication. Future studies should include more unstable fractures in order to characterise their in vivo biomechanics as they are the ones most prone to mechanical complications.
Please cite this article in press as: A.J. Bojan, et al., Trochanteric fracture-implant motion during healing – A radiostereometry (RSA) study, Injury (2018), https://doi.org/10.1016/j.injury.2018.01.005
G Model JINJ 7546 No. of Pages 7
A.J. Bojan et al. / Injury, Int. J. Care Injured xxx (2017) xxx–xxx
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[4]
-1 -2 -3 -4
[5] [6] [7]
-5 [8] Fig. 6. Nail translation in the femoral shaft along the longitudinal nail axis (y), (+) proximal, () distal, n = 8.
Conflict of interest statement The authors, individually and collectively, have no financial, personal or ethical conflicts of interest to report. The study received financial support from IngaBritt och Arne Lundbergs research fund and Swedish Research Council funding for clinical research in medicine and from Stryker GmbH, Schönkirchen, Germany. Funds from Stryker GmbH were solely used to finance the RSA examinations. The funding parties were not involved in the study design, collection, analysis and interpretation of data, the writing of the manuscript or the decision to submit the manuscript for publication.
[9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
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Please cite this article in press as: A.J. Bojan, et al., Trochanteric fracture-implant motion during healing – A radiostereometry (RSA) study, Injury (2018), https://doi.org/10.1016/j.injury.2018.01.005