hip dislocation following sliding hip screw ( shs ) fixation for pertrochanteric extracapsular proximal femur fractures is a very rare complication in absence of infection . gradual impaction and controlled collapse at the fracture site occurs after shs fixation which allows the fracture to stabilize during the postoperative period 1.the most important surgical principle is to ensure adequate fracture reduction . the reduction of the fracture to a slight degree of valgus offers mechanical advantage in creating stability and significantly reduces the chance of fixation failure 1,2 . the optimal position of the shs lag screw should be central / inferior on the anterioposterior radiograph and central on lateral radiograph with an aim of tip apex distance ( tad ) of less than 25  mm to prevent cut out of the implant 3 . there is a high risk of fixation failure in comminuted , unstable pertrochanteric proximal femoral fractures where the lateral support is fractured at the site of lag screw insertion due to excessive medial displacement of the femur . despite this technical complexity , fixation of these unstable pertrochanteric proximal femoral fractures with shs remains a popular choice in the united kingdom and internationally , with evidence from the cochrane database supporting the selection of shs in favor of intramedullary devices 4 . dislocation of the hip following shs fixation has been described in fewer than five cases . melton , yates , and middleton describe two cases of atraumatic posterior dislocation in patients with deficient abductors . both dislocations occurred between 2127  days postoperation and both eventually required girdlestone excision arthroplasty 5 . munjal and krikler also described hip dislocation with an avulsion of the anterior capsule from the trochanteric line 14  days following shs fixation which was managed successfully with an open reduction and soft tissue repair . anterior , atraumatic dislocation of the hip following shs fixation has not been described previously . a 78-year - old female patient was admitted with displaced 4 part pertrochanteric fracture ( ao 31-a2 ) in december 2010 ( fig.1 ) . her comorbidities included dementia and previous cardiac arrest in the month of april in the same year . as per the local rehabilitation protocol , the patient was permitted to mobilize full weight bearing following fixation . however , the patient had poor mobility prior to the fall because of her comorbidities and continued to have poor mobility postoperatively . her hip radiographs were repeated at 2  weeks postoperatively and it showed shs fixation in  situ with impaction at the fracture site , which normally occurs following such dynamic implant fixation . in addition , the x - ray images showed migration of the femoral shaft proximally and medially ( figs.4 and 5 ) . 4 part pertrochanteric extra - articular fracture neck of femur . intraoperative image of shs lag screw position on ap view . intraoperative image of poor fracture reduction and posterior lag screw position . proximal and anterior migration of the femur shaft and collapse at the fracture site at 2  weeks postop . she continued to have poor mobility and therefore the radiographs were repeated at the 6  week postoperative review . there was no evidence of infection at the operative site and her inflammatory markers were within normal limits . serial radiographs over the next 6  months showed progressive hip joint subluxation and at the 6  month review the hip joint had dislocated anteriorly and laterally ( figs.8 and 9 ) . this was further visualized on ct scan , which also demonstrated a mal union of the initial fracture ( figs.10 and 11 ) . hip aspiration showed clear fluid and did not grow any organisms on extended enriched culture . the serial inflammatory markers including c - reactive protein ( crp ) and white blood cell ( wbc ) count were normal over 6  months with highest crp result being 19 at 7  months postoperative period . she underwent revision surgery with calcar bearing uncemented total hip replacement ( figs.12 and 13 ) . intraoperatively ; the surgeon noted a malunited fracture with excessive anteversion of the femoral neck . the patient underwent revision surgery with calcar bearing uncemented hip replacement 7  months after the index operation . following revision the patient was discharged to a residential home where she passed away few months later . sliding hip screw fixation for pertrochanteric fractures of the proximal femur is a popular fixation method as it allows controlled collapse at the fracture site 7 . anatomical or slight valgus reduction of the extracapsular fracture , position of the shs lag screw in the center of the femoral head with tad less than 25  mm are recommended for optimal fixation and to decrease the risk of failure 13.unstable 4 part pertrochanteric fractures are difficult to treat , especially when the lateral support is involved in the fracture or posteromedial weight bearing cortex comminution is present . when there is no lateral cortical support , the shaft of the femur displaces medially reducing the area of bone to bone contact between fragments and may cause delayed union 8 . the selection of the shs for the fixation of this fracture represents standard care in our unit . of these , there is little current evidence that intramedullary nails are superior for pertrochanteric ao 31-a1 or a2 fractures 4 and equivocal evidence that compression plates confer an outcome advantage 9 . trochanteric extension plating has been showed to reduce lateralization of the greater trochanter , and to potentially prevent excessive collapse of the fracture 10 . despite this advantage , hip dislocation following shs fixation for pertrochanteric fractures of the proximal femur is a very rare complication . common features between these posterior dislocations and our case include poor tissue quality and delay to dislocation . in melton , yates , and middleton s cases , abductor tissue quality was poor due to age and parkinson s disease in one patient , and alcoholism in another . in these cases the femoral neck had been fixed in a retroverted position , which would have predisposed to the posterior dislocation 5 . munjal and krikler s case involved another elderly patient who dislocated hip at 2  weeks after surgery , which was noticed with shortened leg and painful hip rotations , without additional trauma . this was a bedbound patient who suffered a dislocation and during open reduction an anterior capsular reattachment was required . there was no indication that the implant had been placed in anteversion or retroversion , and the patient succumbed to chest infection before long term follow - up could be obtained 6 . aside from poor tissue quality and implant positioning , other reports have identified infection and trauma as causes for dislocations following shs fixation of pertrochanteric fractures11 . in our case we were mindful to exclude infection via inflammatory markers and joint aspirate as this would have a significant bearing on management options . it is possible that progressive subluxation is more common than reported , as many patients are unable to complete extended follow - up due to mortality . 12 in japan , where a patient experienced pain following shs fixation of a pertrochanteric fracture . unlike our case , this was revised to a hemiarthroplasty before dislocation occurred and the patient survived to 15  month follow - up with a good functional outcome . a further case of progressive subluxation leading to a posterior dislocation has been described in a 36-year - old patient with a similar unstable fracture , but this appears to be due to the high energy injury and avascular necrosis which was not a feature of this case 13 . poor reduction , poor positioning of the implant , particularly posterior placement of the shs lag screw and torn capsule possibly led to an incongruent hip joint . subsequent weight bearing leading to repetitive stress / trauma on the torn capsule may have led to progression of the subluxation to a dislocation . the surgeon , at the time of revision surgery , noted excessive anteversion of the proximal femur . excessive anteversion of the femoral head puts more stress on the damaged anterior capsule leading to anterior dislocation rather than more common posterior dislocation . unstable 4 part pertrochanteric fractures of the proximal femur without lateral cortical support presents considerable technical problem in treatment and the risk of failure is high with any implant . anatomical or slight valgus reduction and subchondral central position of the lag screw in femoral head are of paramount importance in comminuted 4 part extracapsular fracture nof with osteoporotic bone . the aim should be of tad less than 25  mm for reducing the chance of screw cut out as well as other complications . postoperatively , if the patient is not progressing as per the expectation , high index of suspicion should be maintained for potential complications . careful serial clinical examination of the patient and supplementary investigations should be carried out in all patients including patients with dementia . unstable 4 part pertrochanteric fractures of the proximal femur without lateral cortical support presents considerable technical problem in treatment and the risk of failure is high with any implant . anatomical or slight valgus reduction and subchondral central position of the lag screw in femoral head are of paramount importance in comminuted 4 part extracapsular fracture nof with osteoporotic bone . the aim should be of tad less than 25  mm for reducing the chance of screw cut out as well as other complications . postoperatively , if the patient is not progressing as per the expectation , high index of suspicion should be maintained for potential complications . careful serial clinical examination of the patient and supplementary investigations should be carried out in all patients including patients with dementia .
key clinical messageunstable 4 part pertrochanteric fractures without lateral cortical support presents considerable technical problem in treatment with high risk of failure with any implant . anatomical or slight valgus reduction and subchondral central position of the lag screw ( tad < 25  mm ) reduces the chance of screw cut out and other complications .