Background: The treatment options for displaced femoral neck fracture in elderly are screw fixation, hemiarthroplasty and total hip arthroplasty based primarily on age of the patient. The issues in screw fixation are ideal patient selection, optimal number of screws, optimal screw configuration and positioning inside the head and neck of femur. The problems of screw fixation may be loss of fixation, joint penetration, avascular necrosis of femoral head, nonunion, prolonged rehabilitation period and the need for second surgery in failed cases. We hereby present results of a modified screw fixation technique in femoral neck fractures in patients ≥50 years of age. Materials and Methods: Patients ≥50 years of age (range 50-73 years) who sustained displaced femoral neck fracture and fulfilled the inclusion criteria were enrolled in this prospective study. They were treated with closed reduction under image intensifier control and cannulated cancellous screw fixation. Accurate anatomical reduction was not aimed and a cross sectional contact area of 75% without varus was accepted as good reduction.
Four screws were positioned in four quadrants of femoral head and neck, as parallel and as peripheral as possible. Radiological and functional results were evaluated periodically. Sixty four patients who could complete a minimum followup of two years were analyzed.
I NTRODUCTION Despite ever-increasing literature on hip fracture, there are no authoritative and evidence-based guidelines for the management of displaced intracapsular femoral neck fractures (FNF). A general lack of consensus exists among orthopedic trauma surgeons in the management of these fractures. The decision making in hip fracture treatment depends on age, patient's co-morbidities, pre-fracture mobility status, associated injuries, bone quality, fracture configuration and pre-existing degenerative status of the joint. – The present consensus is that FNF in patients below 60 years of age should be treated with internal fixation, and arthroplasty should be reserved for elderly patients above 80 years, 60 to 80 years old patients with displaced FNF still remain a grey zone. The surgical duration, bleeding, need of blood transfusion, infection and immediate postoperative mortality is considered less with internal fixation compared to arthroplasty.
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However, fixation failure and reoperation rates are higher with internal fixation; thus, majority of orthopedic surgeons prefer prosthetic replacement. – Present guidelines favors total hip arthroplasty (THA) in active, independent patients aged 60 years and primary hemiarthroplasty (HA) for elderly, moribund patients with dependent living status. The functional results of arthroplasty however best done cannot equal that of a united FNF.
Many of the daily functional activities of Asian patients require squatting and sitting cross-legged which cannot be performed with arthroplasty, and thus, every efforts of joint conservation should be aimed for. Fixation for FNF is usually performed with cancellous screws or Dynamic hip screw. The controversial factors in cancellous screw fixation are: method of reduction - open or closed, number of screws used – two, three or four, the configuration of screws – parallel or nonparallel, vertical or triangular fashion, the positioning of screws in the head – center of head or in the periphery and the addition of bone grafting – free fibula or quadratus femoris muscle pedicle grafting. Several biomechanical analytical studies have assessed the stability after simulated fracture fixation on cadaveric femora with variable results.
– The conventional AO fixation using three screws in apex proximal triangular configuration is generally practiced by several surgeons for fixing FNF., In a radiographic review study of patients who underwent cannulated cancellous screw fixation for FNF, six different types of screw configurations were found: Triangular configurations, consisting of two parallel screws with a third screw placed either superiorly, inferiorly, anteriorly or posteriorly; and linear configurations with two or three screws in a vertical line. In a multinational survey of 298 Orthopedic surgeons, it was found that 73% agreed on the use of three cannulated screws and more than half used the triangle with base inferior construct. In a biogeometric study of Indian femurs found that inferior half of femoral neck is narrower than superior half and recommended apex distal configuration for screw fixation.
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Both in experimental and clinical studies, controversies exist regarding the ideal screw fixation method that can provide good stability and good clinical results respectively. The area covered by the fixation device is important in any fracture fixation; more the area on either side of fracture, better is the stability. We felt that, by adding the fourth screw and keeping the screws in peripheral portion of the head and neck, the area of fixation can be easily increased in FNF fixation. Simple geometric projections and calculations revealed a dramatic increase in the volume occupied by the four peripheral screws compared to three screws. With the aim of providing higher success rate in a large group of patients using a single uniform procedure, we started four screw fixation. We report the outcome of four quadrant parallel peripheral screw fixation technique in active, independent patients aged ≥50 years with displaced FNF. M ATERIALS AND M ETHODS One hundred and eighty patients with displaced fracture neck femur aged 50 years or more who were operated for displaced femoral neck fracture in our center from January 2005 to December 2008 were included in the study.
The patient selection criteria for four cannulated screw fixation were: age 50 years or more, displaced fracture neck of femur (Garden Grade III and IV), presence of primary compressile trabeculae in hip anteroposterior (AP) radiograph, no radiographic evidence of pathological fracture, and independently mobile patient without neuromuscular disorder or cognitive impairment and without any metabolic bone disease. There was no upper age limit for patient selection. All patients were prospectively followedup. Sixty four patients who completed a minimum of two year followup were included in the study.
There were 34 females and 30 males. The mean age of patients was 60 years range (range 50-73 years). The mean time interval between injury and surgery was 48 hours (range eight hours-seven days).
Sixty two fractures were subcapital and two were transcervical. Operative procedure FNF reduction was attempted by gentle traction with hip in neutral extension, neutral or 10 to 15° of abduction, neutral or 10° of internal rotation with patient on fracture table.
The reduction was checked in both AP and lateral views of image intensifier. Accurate anatomical reduction was not aimed. A cross sectional contact area of more than 75% without varus deformity was considered as satisfactory reduction. If the reduction was unacceptable, direct manipulation or flexion technique was used. (a) X-ray right hip joint showing garden IV femoral neck fracture, (b and c) anteroposterior and lateral fluoroscopic views showing closed reduction The inferior screws were placed first. The starting point for the inferior screws was usually at the level of lesser trochanter.
Since the lateral surface of femur is convex, the starting point often slips anterior or posterior. To avoid this, a pilot hole was made with 3mm K wire perpendicular to shaft surface in the exact needy location i.e., posterior to mid-lateral cortex at the level of lesser trochanter for inferior posterior (IP) screw. The pilot hole was enlarged with 3.5- or 4-mm drill bit, which was directed in oblique direction as required to make an oval hole in the cortex. A nonthreaded 2-mm guide wire was taken in the hand and pushed into the desired screw trajectory: parallel and just superior to the inferior border of the neck in the anteroposterior view, parallel and just anterior to the posterior border of the neck in the lateral view.
With bigger pilot hole, the guide wire could be easily manipulated in both the planes in the required direction. The guide wire was hand pushed up to head-neck junction. After positioning the guide wire in the desired path, the same was drilled into the head 2mm short of the articular margin.
Tapping of the lateral cortex was done for screw entry. No drilling or tapping was done in the head or neck region.
A 7-mm diameter, 16-mm partially threaded cannulated cancellous screw of adequate length was passed over the guide wire and tightened noting the compression across fracture site. If the neck diameter was smaller, a 6-mm diameter screw was used.
For inferior anterior (IA) screw, pilot drill hole was made exactly at the same level of IP screw, anterior to the mid lateral cortex of femur. The guide wire was passed free hand, parallel to the IP screw in the AP view and parallel and just posterior to the anterior cortex of the neck in the lateral view. The superior posterior (SP) screw was placed just below and parallel to the superior border of the neck in the AP view and parallel to the IP screw in the lateral view.
The superior anterior (SA) screw was placed parallel to the SP screw in the AP plane and parallel to the IA screw in the lateral view. Fluoroscopic views (a-c) showing superior anterior quadrant guide wire and screw In this fashion, all the four quadrants i.e., inferior posterior, inferior anterior, superior posterior and superior anterior quadrants of the femoral heads were stabilized with the neck/trochanter fragment. The screws were placed as much parallel as possible and as peripheral in the neck as possible. We term this fixation as four quadrant parallel peripheral (FQPP) fixation.
The screws could diverge from the neck into the head but convergence of the screws towards head center was not accepted. The mean duration of surgery was 42 minutes (range 30-55 minutes). Peroperative photograpyh showing screws head configuration at entry point in four quadrant fixation Postoperatively, the limb was kept in 10-15° of abduction with knee flexed at 10-20°. Postoperative radiographs were obtained on the first postoperative day. The patient was encouraged to do static quadriceps, terminal knee extension and active assisted/active straight leg raising (SLR) exercises once anesthesia weans off. Patient was advised for opposite side turns in the bed in the immediate postop period.
Twenty four hours after surgery, patient was made to sit up on the bed side and nonweight bearing (NWB) walking was started. On 2 nd postop day, transfer to high seat, mobile western commode was allowed. Patients were discharged from hospital 48-72 hours after surgery.
The patients were followedup clinically and radiologically after three weeks, six weeks, three months, six months, twelve months and after two years. Partial weight bearing (PWB) was allowed after three weeks and full weight bearing (FWB) was allowed after radiographic union was evident. Radiologically fracture union was defined as continuity of at least three cortices in AP and lateral views without any fracture gap. Clinically fracture was considered as healed when there was no local tenderness and patient could do full weight bearing without any support. Stair climbing and hip abductor strengthening exercises were initiated at six weeks. Once the fracture was healed, patients were encouraged to sit on the floor cross legged and to do squatting. The functional outcome was evaluated with a new six-point score.
The one year score was taken as final functional outcome. X-ray left hip joint anteroposterior view (a) showing femoral neck fracture, (b,c) anteroposterior and lateral views showing union after four quadrant parallel peripheral screw fixation Nonanatomical healing or malunion was observed in 45 cases (70%).
Various components of malunion observed were coxa breva ( n = 35), coxa valga ( n = 22), rotational ( n = 18), translational ( n = 12) and coxa vara ( n = 1). If there was cortical reduction at one border of the neck with step in another border, it was considered as rotational malunion.
If there was a cortical step at both superior and inferior borders of neck, it was considered as translational malunion. X-ray hip joint anteroposterior view showing healed femoral neck fracture (a) coxa breva (b) coxa valga (c) rotational malunion (d) translational malunion (e) coxa vara The functional score was excellent in 50 hips (78.2%), good in 13 hips (20.3%) and poor in one hip (1.5%).
The single patient with poor outcome had chondrolysis with progressive femoral head resorption. Another patient had coxa vara with screw penetration.
Both patients had painful hip with antalgic gait. The first patient underwent uncomplicated cemented THA. Screw removal was done in the second patient resulting in complete pain relief. Except for these two patients, implant removal was not done in any other hip. Other than these two patients, no patient had antalgic or Trendelenburg gait.
X-ray hip joint anteroposterior view (a and b) showing head resorption and total hip arthroplasty All patients, except for the THA patient, could do cross legged sitting in the floor and squatting. Cross legged sitting was complete in 48 hips (flexed knee close to the floor) and partial in the rest. Squatting was complete in 54 hips (thigh close to the abdomen) and partial in the rest. All patients could climb stairs. Fifty two patients achieved maximum functional score at the end of 6 months which was maintained in the subsequent followup. Eleven patients continued to show improvement in sitting cross legged and squatting after 6 months that improved their final score at 1 year. D ISCUSSION Since intracapsular fracture neck of femur heals by primary healing, along with stability in coronal and sagittal planes, absolute rotational stability is necessary across the fracture site.
– The proximity of polyaxial hip joint and the location of fracture at junction of lower limb and trunk results in variety of forces acting across the fracture, even in a bed bound patient. The commonly used screw fixation construct in FNF may be imperfect with one or more of technical flaws: Unacceptable reduction (less contact area), lack of parallelism, convergence towards head center, crowding of screws in small area, inadequate screw length, repeated drilling into the head weakening screw purchase and leaving fracture gap. Loading on imperfect mechanical construct can result in uncontrolled collapse, tilting of head into varus, loss of contact, nonunion and screw penetration into the joint. The key therefore is to provide good stable construct that can withstand the “routine” strains, still maintain contact between the fracture ends, provide stability and allow healing. The FQPP construct gives better initial stability, allows better controlled collapse, improves stability with passing time because of increased contact and promotes union. The four screws which are placed in the periphery of the neck act as four struts or pillars and provide excellent stability. Even if the screw purchase is suboptimal in osteoporotic bones, FQPP screws act as internal splints.
The good peripheral or circumferential fixation does not allow the head to settle in varus or any angulations and eliminates the detrimental rotational strain. Since the screws are parallel, they permit controlled collapse maintaining the contact at any point of time. The problems of three screw fixation can be of twofold: the adequacy of fixation especially in older population and ability for controlled collapse. This is especially common when there is lack of parallelism between the three screws and the apex screw is not in the center of two basal screws.
During daily activities, the loads on femoral head alternate anteriorly and posteriorly. If there is less support on the anterior quadrants, loading can result in head tilting into varus anteriorly with screw penetration in the posterior quadrant of the head. If there is less support on the posterior quadrants, loading can result in head tilting into varus posteriorly with screw penetration in the anterior quadrant of the head. In FQPP construct, there is uniform load distribution across all areas of the femoral head. There was varying degrees of screw backout in almost all cases. This could be seen as early as 24 hours after surgery, because patients were allowed to do bed side activities and early SLR exercises.
This controlled collapse or fracture settling is similar to that of distal radius fracture or intertrochanteric fracture and can be considered as a good sign provided the reduction and screws orientation are maintained. In the 70 year old patient who developed femoral head fragmentation and resorption, cemented THA was performed 14 weeks after the index surgery. There were no signs of infection, inflammation and tumor preoperatively. Histo-pathological study of the retrieved loose bony fragments did not show any pathology. We failed to attribute any particular reason for this complication in this case. We have evaluated the clinical outcome with a new six point hip functional score.
The existing scoring systems either do not consider sitting cross legged and squatting or cumbersome for quick clinical application. Assessment at any point of time in the postoperative period should convey the functional status since the results could not be expressed as an improvement over the preoperative score, which is not possible in fracture cases.
This new hip clinical score is based on the routine clinical observations or questions that are done with patient during their clinical visit. Pain, range of motion in the form of sitting cross legged and squatting, functional activity in the form of walking, stair climbing and individual mobility outside the house were assessed. In elderly patients, basic functional assessment may be sufficient rather than advanced functional assessment, as the individual functional capacity is highly variable depending on underlying general fitness. The assessment with the new score is fast and can be done by surgeon or any trained health professional. Being a single digit numeric score, we feel it easily reflects the postsurgical functional results. The results of our study are relevant in general and in particular to Indian population. Many Indian patients come from rural background, where they do physical work even in advanced age.
With healed FNF, “unsupervised” physical work, sitting in floor, squatting and regular life style can be allowed. Unlike replacement arthroplasty, prolonged medical supervision or followup is not necessary. Most of the patients do not have insurance cover, the cost of FNF fixation surgery is less than that of HA or THA. While factors like high cost, lack of surgical expertise, lack of operation theatre facilities and fear for dislocation may force surgeon against THA option, fear for fixation failure and nonunion may force surgeons against fixation option, leaving the lone HA option which is not suitable for all age groups. With high success rate, FQPP can be picked as first choice over THA or HA, especially in the age group between 50 to 70 years. While revising a failed closed screw fixation surgery into THA is almost like a virgin hip surgery, converting painful or failed hemiarthroplasty into THA is difficult and akin to revision THA on several occasions with suboptimal results. The advantages of FQPP technique are many: good clinical outcome, a modification of existing technique which surgeons are familiar, small learning curve, simple routine theatre setup, short surgical time and low expenditure.
The only disadvantage is the initial difficulty in optimal screw positioning. The technique may appear demanding, but with adherence to key surgical steps, it can be mastered easily. The strengths of our study are that it is a prospective study, has relatively large sample population and has adequate followup. The weakness of the study is assessment with a new hip score. Few myths regarding FNF should be addressed. The first and foremost myth is that an accurate anatomical reduction is essential for fracture union and FNF cannot malunite. As shown from this study, anatomical reduction in FNF is neither mandatory for fracture healing nor necessary for satisfactory function.
We achieved anatomical reduction of FNF on the operation table only in 50% cases. However, “near” anatomical healing was seen only in 30% of cases. About 70% of healed fractures had one or more elements of malunion.
This should be viewed as “nonanatomical” healing rather than malunion. The changes in anatomy are increased neck shaft angle, few millimeters of displacement or collapse and few degrees of rotation. In FNF, this nonanatomical healing can hamper some function like full cross legged sitting in the floor and full squatting, but patients can walk comfortably without pain, limp and walking aid. FNF is intracapsular but not intraarticular fracture. The emphasis should be on perfect fixation rather perfect reduction. The tips in getting acceptable reduction in FNF are minimal traction, neutral or mild internal rotation, neutral or mild abduction and understanding the three dimensional anatomy before manipulation. Heavy traction, vigorous manipulation and excessive internal rotation are common pitfalls that prevent reduction.
Except for varus orientation, malreductions like valgus orientation, single cortical step and bicortical steps should be accepted. Repeated manipulations should not be done to reduce the opening of anterior or posterior cortex, which could be easily achieved with screw compression. A cross sectional fracture contact area of 75% or more without coxa vara should be the aim of reduction. The second myth is about the synovial fluid bathing the fracture surfaces resulting in FNF nonunion. Amongst synovial joints the largest synovial fluid volume is that of knee joint. All intraarticular fractures involving femoral condyle and tibial condyle can heal with simple splinting.
While these intraarticular fractures can heal by “secondary” healing without any surgical intervention, FNF requires implant fixation stability for “primary” healing. The third myth is that most if not all FNF results in AVN femoral head.
Vigorous traction and manipulations, multiple un-physiological manipulations and open reduction may result in AVN. We have accepted nonanatomical reduction rather than resorting open reduction in our cases. The fracture per se does not result in AVN at least in older population, where the injury is of low velocity nature and the capsule is intact.
In India, several patients with untreated FNF present late, walking with painless limp. Their X-rays reveal nonunion and neck resorption but not AVN. The average time delay between injury and surgery in our patients was 48 hours. The longest gap was seven days. No patient developed AVN after fixation in our study. In conclusion, four quadrant parallel peripheral screw fixation technique gives good clinical results in displaced femoral neck fractures in large group of patients aged 50 years or above.
This technique, a slight variation of the regular three screw fixation construct, provides excellent stability necessary for primary or endosteal healing of these fractures. With high healing rates, minimal complications and the availability of rescue surgery in the form of prosthetic replacement, this fixation should be attempted in all possible cases of femoral neck fractures in older population.
Results The peak compression occurs at an insertion depth of −3.1 mm, −2.8 mm, 0.9 mm, and 1.5 mm for the Acutrak Mini, Acutrak Standard, Herbert-Whipple, and Synthes screws respectively (insertion depth is positive when the screw is proud above the bone and negative when buried). The compression and insertion torque at a depth of −2 mm were found to be 113 ± 18 N and 0.348 ± 0.052 Nm for the Acutrak Standard, 104 ± 15 N and 0.175 ± 0.008 Nm for the Acutrak Mini, 78 ± 9 N and 0.245 ± 0.006 Nm for the Herbert-Whipple, and 67 ± 2N, 0.233 ± 0.010 Nm for the Synthes headless compression screws. Conclusions All 4 screws generated a sizable amount of compression ( 60 N) over a wide range of insertion depths. The compression at the commonly recommended insertion depth of −2 mm was not significantly different between screws; thus, implant selection should not be based on compression profile alone. Conically shaped screws (Acutrak) generated their peak compression when they were fully buried in the foam whereas the shanked screws (Synthes and Herbert-Whipple) reached peak compression before they were fully inserted. Because insertion torque correlated poorly with compression, surgeons should avoid using tactile judgment of torque as a proxy for compression.
The scaphoid is the most commonly fractured carpal bone, accounting for approximately 60% of all carpal fractures and an estimated incidence of 30 fractures per 100,000 person-years. This injury occurs predominantly in young healthy adults and is associated with a high incidence of delayed union, nonunion, and osteonecrosis owing to the tenuous blood supply to the bone., Management of these injuries was transformed in the 1980s with the introduction of the scaphoid specific Herbert screw and subsequent screw variations. A headless screw generates interfragmentary compression through differential pitches between the leading and the trailing threads. The compression thereby provides rigid internal fixation without the intra-articular prominence of standard headed screws. A headless compression screw (HCS) has become the implant of choice for the internal fixation of displaced and nondisplaced scaphoid fractures., Given the popularity of the HCS, sundry commercial designs have emerged, each with its own variation in thread pitch, shaft diameter, and shape. Despite an abundance of recent papers describing biomechanical testing of these screws in both human cadaver – and polyurethane foam – scaphoid models, the results are discordant, and there is little consensus on optimal screw design. Moreover, the vast majority of studies report peak compression force; however, it is unlikely this force is achieved consistently in clinical practice.
In the absence of a load cell measuring compression, the surgeon must rely on screw position (insertion depth) and tactile feedback from the screwdriver (insertion torque), which may or may not correlate with compression. Rather than using peak compression as the end point to compare screws, the compression in relation to insertion depth and torque (the insertion profile) is of greater clinical interest. Knowledge of such a profile would improve our understanding of the implant, provide a better basis for comparing HCSs, and enable the surgeon to optimize compression. The purpose of this study was to determine the insertion profiles of 4 popular, commercially available HCSs. Using a customized setup, both inter-fragmentary compression and insertion torque were measured simultaneously as the test screws were driven into a polyurethane foam scaphoid bone model.
The goals of our study were to identify the relationship between the compression force, the insertion torque, and the insertion depth; to determine the insertion depth that yields peak compression for each screw; and to measure compression and torque at an insertion depth of −2 mm below the cortex. This depth maximizes screw length while ensuring a buried depth of −2 mm below the articular cartilage and is frequently recommended in the literature. Implants Four commercially available HCSs were tested. All screws were chosen to have similar length (24–25 mm) in order to control for bone purchase. The Acutrak Standard (Acumed, Hillsboro, OR) is a highly polished titanium, conically shaped, self-tapping, fully threaded, cannulated screw with a variable thread pitch spanning the entire screw. It has a distal outer diameter (DOD) of 3.3 mm, and proximal outer diameter (POD) of 4.4 mm. The Acutrak Mini (Acumed, Hillsboro, OR) is a scaled-down version of the Acutrak Standard with DOD and POD of 2.8 mm and 3.5 mm, respectively.
The Synthes 3.0-mm HCS (DePuy Synthes, West Chester, PA) is a cannulated 316L stainless steel, self-drilling, and self-tapping headless screw with DOD and POD of 3.0 mm and 3.5 mm, respectively. A smooth shank that allows for precompression to be applied during screw insertion separates the distal and proximal threads. The Herbert-Whipple HCS (Zimmer, Warsaw, IN) is a modified version of the original Herbert screw with a slightly larger diameter to accommodate cannulation and has self-tapping leading threads. Made of titanium (Ti-6AI-4V alloy), the DOD and POD are 3 mm and 3.85 mm, respectively, separated by a smooth 2.5-mm-diameter shank between proximal and distal threads. The Acutrak screws generate compression through the combination of a conically shaped shaft and variable thread pitch along the screw whereas the shanked screws create compression with 2 constant but different thread pitches at either end of the screw. Scaphoid bone model In order to mitigate testing variability between samples, a rigid polyurethane foam model (1522-03, Pacific Research Laboratories, Vashon, WA) was used to simulate the scaphoid fracture.
The biomechanical properties of the foam are well controlled and were selected to best approximate scaphoid cancellous bone of a young adult,—comprising a density of 0.32 g/mL and compressive, tensile, and shear moduli of 210 MPa, 284 MPa, and 49 MPa, respectively. The foam was machined by computer numerical control into 32 mm × 32 mm × 35 mm blocks, and a diamond saw was used to create a linear osteotomy 12 mm from the surface. A 1-mm layer of denser foam (0.64 g/mL) was laminated to the surface to represent cortical bone. Testing procedure The test setup is shown in. A custom testing jig was used to hold the polyurethane foam blocks and prevent rotation during screw insertion.
The blocks were free to slide vertically within the clamps as compression was applied. In order to minimize the displacement of the simulated fractures, an ultra-thin load sensor was employed.
The FlexiForce (A201, Tekscan, Boston, MA) is a 0.13-mm-thick piezoresistive force sensor printed onto a flexible circuit board. A 5-mm hole between 2 thin metal washers was placed at the center of the sensing head in order to accommodate a screw. The load sensor was then sandwiched between 2 foam blocks, and the HCS under test was placed through the center of the sensor. The total displacement created by the load sensing apparatus was less than 1 mm. A Photograph. The HCSs were inserted according to their respective manufacturer’s guidelines into an intact, new foam block. A precompression force of 70 to 80 N was applied to the Synthes screws using the manufacturer’s specific compression sleeve and screwdriver.
A torque meter (Imada, Northbrook, IL) was used to advance the screws in half-turn intervals while recording the insertion torque, insertion depth, and interfragmentary compression. A time delay of 10 seconds between intervals was used to ensure stable measurements. Insertion depth was measured by digital caliper to the nearest 0.1 mm and defined as the distance from the proximal tip of the screw to the surface of the foam (positive when the screw was proud and negative when the screw was buried in the foam). Each test was stopped after a sustained drop in compression was observed (representing loss of fixation). The experiment was repeated 5 times for each HCS type for a total of 20 tests.
Statistical methods Compression and insertion torque profiles were generated for each trial by synchronizing the data and plotting them versus screw insertion depth. Composite plots taking the mean of the 5 trials were also generated with error bars representing the standard error of the mean. Descriptive statistics and 1-way analysis of variance were used to analyze the compression and torque values. Preliminary validation of the experimental setup yielded compression measurements with a standard deviation of approximately 20 to 25 N. In order to have a standard error of less than 10% (10 N assuming compression forces of approximately 100 N), a sample size of 5 was needed.
Composite profiles for interfragmentary compression (solid lines) and insertion torque (dashed lines). The error bars represent the standard error of the mean. Insertion depth is the distance between the proximal tip of the screw and the surface of the bone (foam). Vertical hashed lines represent the screw position when the proximal tip is flush (0 mm) and buried (−2 mm) below the cortex. The compression and insertion torque at a depth of −2 mm below the surface (recommended insertion depth ), from greatest to least compression, were found to be from the Acutrak Standard (113 ± 18 N, 0.348 ± 0.052 Nm), Acutrak Mini (104 ± 15 N, 0.175 ± 0.008 Nm), Herbert-Whipple (78 ± 9 N, 0.245 ± 0.006 Nm), and Synthes (67 ± 2N, 0.233 ± 0.010 Nm) HCSs. Analysis of variance demonstrated no significant difference in compression or torque between any of the screws ( P.05).
DISCUSSION Bone quality, fracture geometry, and patient comorbidities are uncontrollable factors affecting healing; however, the choice of implant may be at the surgeon’s discretion. For this reason, there has been a flurry of studies evaluating the biomechanical advantage of 1 HCS over the other. Investigations have looked at decay in peak compression over time, compression after screw reinsertion, compression along the length of the screw, effect of central versus eccentric screw placement, – pull-apart force, and failure under cyclical loading., The most widely studied parameter, however, is interfragmentary compression.,–, Overall there is variability in absolute results between studies whereas the relative comparison between screws consistently shows that second-generation HCSs, such as the 4 screws in this study, outperform the original Herbert screw.
Why the emphasis on compression? Theoretically, the advantages are clear. Compression improves fracture stability and limits strain and shear along the fracture site, thereby facilitating primary bone healing. Although many studies highlight the need to maximize compression, the absolute value needed for sufficient scaphoid fracture healing is unknown.
Clinically, the impact of compression is even less certain. A retrospective study by Gregory and colleagues demonstrated no significant difference in union rate or time to union in patients treated for scaphoid delayed and nonunions with either Herbert or Acutrak HCSs. In a similar but larger study by Oduwole et al, union rates were higher with the Acutrak group; however, the results may have been confounded by improved screw placement and surgeon experience in the Acutrak group. Finally, the longevity of applied compression is also unknown. Gruszka et al recently showed that compression wanes significantly over time, dropping by 39% to 55% after 12 hours. Given the lack of prospective clinical comparison between HCSs, the link between compression and clinical outcome remains unproven and the amount of compression needed for sufficient bone healing is unknown.
Those selecting implants should, therefore, consider practical factors such as cost, ease, and familiarity with the screw rather than compression alone. Using peak compression as a basis of comparing screws may be misguiding. Our study showed that peak compression often occurred at insertion depths outside the clinically useful range (eg, when the screw was either proud or very deep).
In fact, peak compression was achieved at an average distance of 2.1 mm away from the ideal insertion depth recommended in the literature and by manufacturers. In practice, surgeons must rely on proxies for compression, chiefly screw position (insertion depth) by direct visualization or fluoroscopy and tactile feedback from the screwdriver (insertion torque). It, therefore, makes sense to study compression as a function of these 2 variables. Furthermore, interfragmentary compression as a profile, over a wide range of insertion depths, is more meaningful than at a single point in time or depth because placement will vary from patient to patient.
The profiles of the 2 Acutrak screws differed considerably from that of the Synthes and Herbert-Whipple implants. Furthermore, the variability in compression and torque was greater in the Acutrak groups. Although a larger sample size may have led to a significant difference between screws, it is unlikely this would be clinically significant because the SD was only 2% to 16% of the mean. The conically shaped, fully threaded design of the Acutrak perpetually generates compression as seen by a positive slope in the compression profile throughout most of insertion. Both the standard and the mini versions demonstrated a loss of compression as the trailing end of the screw leaves the cortex (1 mm dense foam layer); however, the compression is quickly regained as the threads tap into the deeper (cancellous) foam. The peak compression in these screws occurs at approximately −3 mm below the cortex, which is very close to the recommended and clinically desired safe insertion depth.
Loss of compression at deeper insertion ensues as the leading threads begin to strip; however, this occurs beyond a clinically targeted screw depth. Insertion torque increased continuously with screw insertion, and the values for the Acutrak Standard were similar to those previously reported by Pensy et al. The torque was approximately 50% higher for the Acutrak Standard screw than for the Mini, which is commensurate to the larger diameter and increased surface area with bone. The risk of rotating the proximal fragment and losing the reduction is greater when high insertion torque is required to advance the screw. Intuitively, it would appear that insertion torque directly correlates with compression force owing to the increased friction between bone and implant.
Although both parameters increased together, the correlation between them was poor. Insertion torque rose continuously with screw insertion, even when compression was lost. The Synthes and Herbert-Whipple screws have a shanked design and generated their peak compression before the implants were fully buried in foam.
Furthermore, some of the precompression applied to the Synthes screw was lost during insertion despite the differential thread pitch between ends, which is designed to augment compression throughout insertion. Nonetheless, both shanked screws demonstrated relatively stable insertion profiles, offering consistent compression over a wide range of insertion depths. The effect of traversing the denser foam (cortex) was less evident in these profiles. Similar to the Acutrak screws, the torque correlated poorly to compression and continued to increase even when compression was lost.
Limitations of this study include ex vivo evaluation, analysis limited to compression and insertion torque, and fracture simulation using a perfectly linear and perpendicular plane to the screw. These limitations were needed to enable repeatable and reliable testing of the implants and are unlikely to affect the external validity of our findings. Based on the insertion profiles generated in this study, several important findings were made. First, conically shaped HCSs generated their peak compression when they were fully buried in the foam whereas the shanked HCSs reached peak compression before they were fully inserted. This finding suggests that the Acutrak screws should be inserted at least 2 mm below the cortex and the Synthes and Herbert-Whipple would provide more optimal compression when the trailing end remains roughly flush with the cortex. Second, insertion torque was shown to correlate poorly with compression, peaking far after the point of maximum compression.
Unlike standard, uniformly threaded screws, 1 end of the compression screw may be loose while the other is firmly engaged in bone, thereby generating high insertion torque with little compression. Surgeons should, therefore, avoid using tactile judgment of torque as a proxy for compression. Third, all 4 screws tested generated a sizable amount of compression ( 60 N or 6 kg) over a wide range of insertion depths. The actual compression at an insertion depth of 2 mm below the cortex was not significantly different between HCSs.
Given that we do not know how much compression is required for bone healing and that all 4 screws provided comparable amounts of compression, implant selection should consider practical factors such as the ability to achieve adequate reduction, good implant placement, and familiarity with the implant specific instrumentation rather than compression alone.