3D Computer Modeling Analysis of the Surface
Area and Thread Volume of the Common Sliding
*Andrew Gardner1, Dominic Eggbeer2, Micheal Ryan3 and Paul Y F Lee1,4
1South Wales Orthopaedics research network, Welshbone, Cardiff, United Kingdom
2PDR Surgical Prosthetic Design, Cardiff Metropolitan University, 200 Western Avenue, Cardiff, United Kingdom
3Engaged Manufacturing, Cardiff Business School, Cardiff university, Queen’s Buildings, 5, The Parade, Cardiff, UK
4School of Engineering, Cardiff University, Queen’s Buildings, 5, The Parade, Cardiff, UK
Submission: February 12, 2017; Published: February 27, 2017
*Corresponding author: Andrew Gardner, Research fellow, South Wales Orthopaedics research network, Welshbone, Cardiff, United Kingdom, Tel:+61 432155518;Email:firstname.lastname@example.org
How to cite this article:Gardner A, Eggbeer D, Ryan M, Lee PYF. 3D Computer Modeling Analysis of the Surface Area and Thread Volume of the Common Sliding Hip Screw. Ortho & Rheum Open Access 2017; 5(1): 555653. DOI: 10.19080/OROAJ.2017.05.555653
Introduction: The Sliding Hip Screw (SHS) is a tried and tested method used to treat intertrochanteric hip fractures. Lag screw cut-out is a major complication of SHS fixation and accepted risk factors are often re-evaluated. We aim to demonstrate a new hypothesis of cut-out using 3D reconstructive modelling. We propose that the risk of cut-out is much greater in lag screws with a smaller thread surface area and larger thread volume.
Method: The four commonly used SHS lag screw systems (Biomet, Synthes, Stryker and Smith & Nephew) were CT scanned and the images processed and turned into 3D computer models for further analysis.
Results: There were significan differences between the surface area and thread volumes of the lag screws between manufacturers ranging from 67.0mm2 - 347.4 mm2 and 166.376mm3 - 225.687mm3 respectively.
Discussion: Assuming the consistency of bone is equal, we propose that the risk of cut-out is much greater in lag screws with a smaller surface area to thread volume ratio (SA:TV). The reamer design of lag screws is also discussed.
Conclusion: Although there are many non-modifiable risk factors that contribute to screw cut-out, selecting the right raw materials for SHS fixation will have a favorable impact on the overall risk.
The Sliding hip screw (SHS) is the most commonly used device in the fixation of intertrochanteric hip fractures . Lag screw cut out is a common post operative complication which accounts for up to 84%  of all fixation failures and occurs at rates between 1%-6.8% [3-5]. Lag screw depth and placement, therefore, has been quantifiable using the tip-apex distance (TAD)  and the ratio method  respectively. The TAD has been shown to be the most important predictive factor of screw cut out and in more recent studies has been stated as greater than 15mm . The position of the screw in the femoral head remains an important factor and most studies agree that anterior and superior screw placement should be avoided [7-12]. Peripheral placement of the screw has also shown to increase the TAD .
Despite widespread agreement that the TAD is the most important factor in screw cut out it does not take into account size of the femoral head or bone density and therefore may not be an ideal concept . Patient, implant and mechanical factors contributing to screw cut out have been extensively discussed in the literature and it is widely accepted that there are other factors yet to be considered. The current paper will discuss other biomechanical factors involved in screw cut out.
The SHS has a design specification (size, shape, weight, thread pattern) determined by its manufacturer. Different sizes and thread patterns suggests that varying amounts of screw would be engaged with bone at any one time and varied volumes of bone removed or compacted on reaming. We propose that the greater the surface area (SA) of the thread end (TE) of the lag screw, the better fixation and the lower the probability of screw
cut-out. We aim to reinforce our hypothesis using 3-dimensional
(3D) remodeling of lag screws.
There are four commonly used SHS lag screw systems,
Biomet, Synthes, Stryker and Smith & Nephew (SN), all of which
have their own specifications. In order to quantitatively evaluate
the SA of each screw type, each screw was Computer Tomography
(CT) scanned with a pixel resolution of 0.346mm, slice thickness
of 0.625mm and using slice increments of 0.312mm (Light Speed
VCT, GE Medical, UK). The data were imported into the medical
image processing software, Mimics (V16, Materialise, Belgium).
This software enables CT images to be processed and turned into
3D computer models for further analysis. The ‘Thresholding’ tool
was used to create a ‘mask’ of each screw based on the material
density in Houns field units.
The screws showed as bright white (dense) material with a
light grey halo (caused by the partial pixel effect) against a black
background. A lower value of 2270 and higher value of 3071
units was chosen based upon visual analysis of how well the
screw details were portrayed; values outside of this range either
included a ‘halo’ effect around the screw or resulted in known
details, such as the threads, from not being included in the
‘mask’. A separate ‘mask’ for each screw type was created using
the ‘Region Grow’ tool and they were then labelled according to
the manufacturer details.
The ‘Calculate 3D from Mask’ tool was used to create a 3D
model of each screw. These were aligned to be parallel to each
other and in a line on screen, making it easy to compare the
profiles iles (Figure 1). Once aligned, a new ‘mask’ of the screws
was created using the ‘Calculate Mask from 3D’ tool. A new mask
of each screw was created using the ‘Region Grow’ tool and the
Core End (CE) was cropped off the mask using the ‘Crop Mask’
tool – this left just the Thread End (TE). A new 3D model of the
TE sections of each screw was created to allow for analysis of the
SA and thread volume (TV).
The total SA of the TE of all four lag screws is presented in
Table 1. The largest SA of lag screw was manufactured by SN
(1,344.071mm2± 154.811, n=4) and the smallest SA by Biomet
(996.6471mm2± 154.811, n=4). The mean SA was 1185.069mm2
(± 154.811, n=4).
The SA of all four lag screws were compared against each
other individually. The results of this are summarised in Table
2. The largest difference in SA of the TE was seen between SN
and Biomet (347.423mm2 [34.9%] ± 113.6, n=4). The smallest
difference in SA was seen between Stryker and SN (67.0mm2
[5.0%] ±113.6, n=4). The mean variation between manufacturers
was 251.2mm2 (25.2%, ± 113.6, n=4).
The thread volume (TV) of the four lag screws are
summarised in Table 6. The largest TV was calculated from the
lag screw manufactured by Biomet (218.559mm3 ± 32.037, n=4)
and smallest by SN (166.376mm3± 32.037, n=4).
The most common complication associated with SHS
fixation is lag screw cut-out . The aim of the current study was
to provide evidence of other factors that might contribute to lag
screw cut-out following SHS fixation. Assuming the consistency
of bone is equal, we proposed that the SA of the screw in contact
with bone has a significant impact on the stability of fixation and
the probability of cut-out. A larger SA of screw will correspond
to a greater proportion of bone engaged and reduce the load
per unit area, thus will have a smaller risk of cutting-out. From
the 3D reconstruction analysis, we have been able to show
significant differences between the SA of the TE of the lag screws
between the four different manufacturers, in some cases up to
35%. However, these figures only contribute a single factor to the
overall risk of cut-out due to the large number of other factors.
It has been acknowledged in the literature that factors such as
degree of osteoporosis, fracture instability, anatomic reduction,
location of the screw in the femoral head and the experience of
the surgeon can contribute to overall outcome of DHS fixations
[7,15-17]. Of these, perhaps only some factors are modi fiable
(Table 4) and therefore should be targeted in order to reduce
the risk of complications associated with DHS cut out. There are
large series of clinical data suggesting that the position of the lag
screw in the femoral head, quantity fied by the TAD  and its
ratio , is one of the major determinants of screw cut out.
In line with the proposal of our study, the SA of lag screw
is a factor that can easily be controlled by the selection of
the manufacture and it is easily repeatable and will not vary
according to the experience of the surgeon.
Although the SA of lag screws is important to predict bone
contact, the size and design of the reamers must also be take
into account. Depending on the specific design of the reamer and
its speed of reaming, varying volumes of bone will be removed
or compacted into the femoral head, potentially altering its
density and bio-mechanical characteristics. The sharpness of
the reamers as well as the surgical reaming technique would
also influence the quality of the hole, these factors vary in each
individual case (Table 5) summaries.
Figure 2 the mechanical factors that can influence the fixation
of the lag screw. The design of the reamer and corresponding
lag screw thread design is defined by a number of different
components: pitch angle, thread pitch, root diameter, thread
diameter and thread length (Figure 2). When comparing the lag
screw specifications between manufactures [18-20], the thread
length and thread pitch was seen to be fairly consistent. There
was, however, some variability in pitch angle (PA). Considering
only biomechanical factors in good quality bone, when thread
pitch remains constant, it is the PA that influences the volume
of bone removed (thread volume). As the PA increases so does
the area, in cross-section (Figure 3), corresponding to a greater
thread volume (TV).
For example, the S&N lag screw design had the smallest PA
and thus will correlate with a smaller volume within its thread
(Table 5). Conversely, Biomet, S&N and Stryker screws all have
a larger PA that increases the SA and TV as well as providing
better tensional strength to the screw. This would also reduce
the chance of screw breakage during removal.
In theory, a larger pitch size will capture a large amount
of bone which should provide a better pull out strength.
Experimentally, it has been shown that a greater pitch angle in
pedicle screws provides an increase in pull out force . As
mentioned above, a greater pitch angle corresponds to a larger
volume of bone being removed on reaming and therefore, a
greater pull out strength. Quantifying the strength of screw
engagement with regard to the above factors is beyond the
scope of this paper but is something to consider as an additional
variable to overall outcome.
The lag screw on the SHS, however, is not designed to prevent
pull out but the mode of failure is usually superior lateral cut
out. Thus, the SA of the screw would have a more practical value
as compared to the pitch size in this context as the SA would
represent the area of loading, therefore increased SA contact
between screw and bone will reduce the forces transmitted
per unit area. In addition to this, when considering the cohort
of patients in which the SHS is used, removing large volumes of
bone in elderly osteoporotic patients would further de-stabilise
the fixation. Therefore, a screw that that removes the least
amount of bone would be more favourable. Taking into account
the significant differences in SA between screw manufacturers and the importance of keeping good quality bone insitu, we
propose that a lag screw with a greater Surface Area (SA) to
Thread Volume (TV) ratio (SA:TV) would provide a more stable
fixation and reduce the probability of lag screw cut out. From our
analysis, the company that manufactured a lag screw with the
largest SA and smallest volume of bone removed after reaming,
i.e large SA:TV ratio, was S&N (SA:TV = 8.079). In contrast, the
lag screws with a small SA and large volume of removed bone,
i.e small SA:TV ratio, was manufactured by synthesis (SA: TV=
4.976), Biomet (SA: TV= 4.560) and Stryker (SA:TV= 5.679)
There has been many different factors documented in the
literatures relating to SHS lag screw cut out and its mode of failure,
some of the risk factors are well established [11,18]. However
there has not been any report looking at the characteristics
and SA of the lag screws between manufacturers. Although we
cannot adjust for the experience of the surgeon, fracture pattern
and quality of bone, we can be selective in the implant we use,
resulting in a more favourable risk calculation for screw cut-out.
We believe a SA:TV ratio quantifies an established complication
(cut-out) of DHS fixation and specifically addresses the biomechanical
The limitation of this study is that it is based on a computer
model of the lag screws. Further bio-mechanical and clinical
outcome studies need to be conducted to look at cut-out rates
against SA of the different lag screws.
The size, pitch and thread of lag screws are different between
manufacturers. These components will influence the contact SA
between the implant bone interface, affect the load per unit area
to the surrounding bone, de fine the volume of bone removed
on reaming and ultimately influence cut out. It is, therefore,
important for surgeons to understand the difference between
manufacturer specifications as well as the cost of the implant.