대한정형외과학회지 : 제 44 권제 1 호 2009 J Korean Orthop Assoc 2009; 44: 1-7 불안정성골반골절의치료시경피적천장골나사못의수와위치에대한비교 김원유ㆍ지종훈ㆍ권오수ㆍ박상은ㆍ김영율ㆍ문창윤 가톨릭대학교의과대학대전성모병원정형외과학교실 Comparison with Number and Position of Percutaneous Iliosacral Screws as Treatment of Unstable Pelvic Fracture Weon-Yoo Kim, M.D., Jong-Hun Ji, M.D., O-Su Kwon, M.D., Sang-Eun Park, M.D., Young-Yul Kim, M.D., and Chang-Yun Moon, M.D. Department of Orthopedic Surgery, Daejeon St. Mary's Hospital, The Catholic University of Korea College of Medicine, Daejeon, Korea Purpose: A closed reduction of the posterior arch and percutaneous fixation with S1 and S2 iliosacral (IS) screw was performed on an unstable pelvis fracture with a disruption of the sacroiliac complex. The radiological and clinical results were analyzed according the number of screws and their position. Materials and Methods: Of 31 cases with an unstable pelvis fracture involving the sacral complex, classified as Tile type C (AO/OTA), 16 and 15 cases were treated with one S1 screw fixation and two screws fixation into S1 and S2, respectively, using a percutaneous fixation technique. The patients were followed up for a minimum of 12 months and the radiological and clinical outcomes were analyzed statistically using the Majeed score and SF-36. Results: Five cases of screw displacement occurred in the one screw fixation group. On the other hand, there was no screw displacement in the two screws fixation group after a mean follow-up of 40.2 months. In the case of a narrow safe zone (iliac cortical density, ICD), it is impossible to fix with two S1 screws. However, in these patients, good clinical results were achieved with S1 and S2 were achieved with S1 and S2 screw without complications. Conclusion: The technique of two screws fixation is an efficient and reliable method for reducing and fixing the unstable pelvic ring disruptions. Additional S2 screw fixation is recommended for patients with a narrow ICD. Key Words: Unstable pelvis fracture, Percutaneous iliosacral screw fixation, Number and position of screws 서론골반환후반부의골절과천장관절의이개를동반하는불안정한골반환골절의치료원칙은일반장관골골절의치료와같이골절을만족할만한위치로정복한후에견고한내고정을시행하여조기에재활치료를할수있도록하는것이다 2,4,16). 불안정골반환골절시후방골반환의견고한내고정으로써천장관절의불유합및부정유 합으로야기되는천장관절부위의동통, 하지부동등의합병증을예방할수있다 2,4). 불안정성골반환골절의수술적치료시내고정및외고정에대한여러가지방법들이대두되고있으며, 특히골반환의후방분리시천장관절고정에대해경피적나사못, 금속판, 긴장강선이나천추막대를이용한고정등다양한방법들이소개되어있다 1,8,14). 이러한고정방법들간의강도나내구성을비교 통신저자 : 문창윤대전시중구대흥동 520-2 가톨릭대학교대전성모병원정형외과 TEL: 042-220-9530 ㆍ FAX: 042-221-0429 E-mail: osmcy@naver.com Address reprint requests to Chang-Yun Moon, M.D. Department of Orthopedic Surgery, Daejeon St. Mary's Hospital, 520-2, Daeheung-dong, Jung-gu, Daejeon 301-012, Korea Tel: +82-42-220-9530, Fax: +82-42-221-0429 E-mail: osmcy@naver.com 1
2 김원유ㆍ지종훈ㆍ권오수외 3 인 분석한연구들도보고되고있으며 11,18) 골반환의후방부를고정하는방법의하나로제 1 천추에천장골나사못으로고정하는방법이권장되고있다 18,20). 제 1 천추에나사못고정만으로임상적으로결과가좋다는보고들이있으며, 최근제 2 천추에나사못고정을시행하는경우에관한보고들도나오고있다. 하지만제 2 천추의나사못고정은신경손상의위험때문에술기에어려움이있어제 1 천추에하나의나사못으로고정하는경우가더흔하며, 천장관절의나사못고정시고정방법의차이가임상적결과와어떠한연관이있는지에대한보고는흔하지않다. 이에저자들은불안정골반환골절에서후방도달에의한경피적나사못고정시나사못의수와위치에대한방사선학적결과와임상적결과를평가하기위해 2000 년 5월부터 2006년 12월까지본원에서무작위로전향적인수술적치료를시행한불안정성골반골절 31예를문헌고찰과함께보고하고자한다. 대상및방법골반골절의분류에여러가지가제시되었지만저자들은 Tile의분류를이용하였으며 26), 동반손상이있는경우전방부고정은외고정장치및내고정장치를시행하였고전방부고정방법과는상관없이후방고정의방법에따른비교만을시행하고자하였다. 본원정형외과에 2000 년 5월부터 2006년 12월까지천추골절, 천장관절의이개와천장관절의이개를동반한장골골절과같은 Tile 분류 C의불안정성골반골절로내원한환자를무작위로전향적으로수술적치료를시행한 31예를대상으로하였으며, 술전단순방사선사진과컴퓨터단층촬영을통해천골익의경사각이작은경우안전영역이좁은것으로판단하여, 두개의나사못고정시 S1의안전영역이좁은경우에 S2에나사못고정을하였다. 무작위로초기에시행한 16예에서는 S1에한개의나사못을, 3예에서는 S1 에두개의나사못을, 후반 12예에서는 S1과 S2에각각한개씩의나사못을이용한경피적고정술을시행하고추시관찰을시행하였다. 남성이 14, 여성이 17명이었으며, 연령분포는 23세에서 81세까지로평균연령은 55세였다. 평균수술시간은 34.4분이었다. 전방부에동반손상이있는경우전방고정은내고정및외고정을하였으며본교실에서연구했던결과에따르면전방외고정과후방고정을한경우와전방내고정과후방고정을한경우생 역학적차이는없으나, 단지전방외고정의경우외고정기에의한불편감, 핀감염및불충분한정복의합병증이있어 11), 본연구에서는개방성골절이나생명이위독한위중한상태일경우를제외하고전방내고정을위주로시행하려하였다. 본연구에서는각군에서각각 2예씩의경우만외고정을, 나머지 27예는내고정을사용한예에서대상을선택하였다. 외측압박손상에의한 crescent 골절이흔하지않게보고되고있으나본연구에서는세분하지않고후방천장관절복합체의안정성에기인한 Tile 에의한분류법에기준하여천장관절의분리가있거나천골또는장골의골절이동반되면서천장관절의이개가있는경우즉 Type C손상환자를대상으로하였다. Crescent 골절에대하여는추후연구가필요한것으로생각된다. 나사못고정은환자를앙와위로하여영상증폭장치투시하에골절부위의정복정도를확인유지하면서측면상을통해제 1 천추의위치를확인하고유도핀을골절선으로통과시켜장골에서천골체까지삽입하여그위치를영상투시하에전후상뿐만아니라 inlet, outlet view 를통해확인하고핀이천골체의앞이나척수강으로의탈출이없음을확인한후핀위로나사못을삽입하였다. 해부학적정복여부는수술후촬영한방사선사진을분석하였으며방사선학적평가는술후최소 12개월이후에방사선학적변화로분석하고나사못의해리가있는경우불유합으로판정하였고, 이에대한환자의기능적평가는수술후평균 40.2개월 (12-76개월) 에전화상담을통해 Majeed score 12) 와 SF-36 결과에대해 SPSS- 12.0 program 을이용하였고, 통계방법으로는 ANOVA 를이용하였고, p값이 0.05 보다적은경우유의한것으로평가하였다. 결과경피적나사못을사용한천장관절고정술에서해부학적인수술적정복후평균 40.2개월 (12-76개월) 최종추시시 S1에하나의나사못을사용한경우 5예 (30%) 에서는나사못의전위및천장관절의불유합소견이보여동통이심한 4예에서금속판과나사못을이용한내고정술로천장관절유합술을시행하였고, 나머지 1예는동통이심하지않아추시관찰중이다. 두개의나사못을사용한경우에서는나사못의전위가관찰되지않았다. Majeed에의한골반골절의임상적등급분류에서하나의나사못으
불안정성골반골절의치료시경피적천장골나사못의수와위치에대한비교 3 로고정한군의경우만족이상 (excellent, good) 이 9 예 (53 %), 두개의나사못으로고정한군의경우 14예 (93 %) 였고, SF-36 에서도한개의나사못을사용한군은 60 점, 두개의나사못고정군은 S1에두개의나사못을사용한경우 84점, S1과 S2에나사못을사용한경우 92 점으로두개의나사못을사용한군에서통계학적으로유의하게더만족스러운결과를보였다 (p<0.05)(table 1). 신경손상이나감염등의합병증은발생하지않았다. 1. 증례 1 39세남자환자로 Type C 불안정성골반골절및천추골절, 좌측상하치골절및천장관절의이개가관찰되었다 (Fig. 1A). 제 1 천추와제 2 천추에경피적나사못을이용한고정술과외고정장치를이용한고정술을시행하였고 (Fig. 1B), 최종추시사진상안정된유합을보이고 (Fig. 1C), Majeed score 는 88점으로매우만족에해당하였다. 2. 증례 2 59세남자환자로 Tile C 골절로단순방사선사진 (Fig. 2A) 과컴퓨터단층촬영상 (Fig. 2B, C) 우측비구 Table 1. Majeed Grade of Each Case Excellent Good Fair Poor S1 2 7 S1 & S1 2 1 S1 & S2 8 3 1 골절, 양측치골골절과천골골절및천장관절이개가관찰되었다. 비구골절에대해금속판을이용한내고정술, 치골골절에대해나사못고정술, 천장관절이개에대해서는좁은안정영역으로인하여제 1 천추에하나의나사못을이용한내고정술을시행하였으나 (Fig. 2D), 추시관찰상제 1 천추나사못의해리가관찰되었고 (Fig. 2E) 환자는둔부에동통을호소하여금속판과나사못을이용한내고정술로천장관절유합술을시행하였다 (Fig. 2F). 최종추시사진에서도잘유지되고있는소견을보였으며, Majeed score는 80점으로만족에해당하였다. 3. 증례 3 39세남자환자로제 2 천추좌측천골골절과천장관절이개및치골결합부위분리를동반한불안정성골반골절에대해 (Fig. 3A) C-arm 투시하에, 제 1 천추와제 2 천추에나사못을이용한내고정술을시행하였으며제 2 천추우측천골골절은골절부분쇄가있어서상대적으로긴골편간나사못을이용하여먼저고정하였고좌측은나사못간의충돌을피하기위하여짧은나사못으로내고정하였다 (Fig. 3B). 또, 치골결합부위는전방금속판을이용한내고정술을시행하였다 (Fig. 3C). 최종추시관찰에서 Majeed score 는 92점으로매우만족에해당하였다. 고찰천장관절은인체의관절중가장강하고안정된가동관절이다. 여러인대중천장인대가가장안정성에중요 Fig. 1. (A) A 39 year-old male patient who has unstable pelvic fracture with sacral fracture including left superior and inferior rami and sacroiliac joint disruption. (B) External fixator and percutaneous SI screws fixation on S1 and S2 were performed. (C) Last follow up X-ray shows stable union.
4 김원유ㆍ지종훈ㆍ권오수외 3 인 Fig. 2. (A-C) A 59 year-old male patient who has unstable pelvic fracture with right acetabular fracture, left ramus fracture, sacral fracture and sacroiliac joint disruption. (D) Plate fixation on acetabular fracture, cannulated screw on left ramus fracture and percutaneous IS screw fixation on S1 due to narrow safe zone. (E) Follow up x-ray shows displacement of IS screw and patient complained buttock pain. (F) Second operation as Lt SI joint fusion with plate and screws fixation was performed. 한요소이며, 이천장인대는다시전방, 골간및후방인대로구분되며강한외력에의한심한손상으로천장인대의파열에의해불안정성을유발한다 10,25,27). Kellam 등 8) 은불안정골반붕괴의예후는천장관절정복의정확도와골반환의안정성에의존한다고하였다. 또한불유합과부정유합, 참을수없는만성통증은불안정성골반환골절-탈구와연관된합병증이다 17). 내고정은외고정에비해견고한고정이가능하며역학적연구에의하면편측또는양측불안정성손상시전방외고정만으로는견고한고정이불가능하다. 따라서불안정골반골절시전방은외고정및내고정을실시하되후방의손상은내고정하는것이가장견고하다 2-4,11). 후방고정에대한다양한방법들이알려져있지만최근영상증폭장치하에경피적천장관절나사못고정술과전방천장관절금속판고정술, 전장골압박막대등이소개되어있다 5,6,12,23-26). Letournel이나사못고정을처음소개한이후 Matta 와 Saucedo는복와위에서 14) 관혈적정복후대좌골절흔에손가락을대고 drill 에의한전방구조물의손상을보호하면서나사를천골익에고정하는방법을, Routt 등 20,21) 은 앙와위에서영상증폭장치하에서도수정복혹은관혈적정복후장골익에수직으로제 1 천추혹은제 2 천추체로천장관절을통과하여 1-2개의나사를고정하는방법을제시하였다. Kim 등 9) 은제 1 천추와제 2 천추에고정하는방법을소개하여좋은결과를보고하였다. 반면 Keating 등 7) 은 38명의수직적불안정골반손상환자에게천장관절나사못고정의효과에대해연구하여 16명의천장관절탈구중 7명에서높은불유합의발생을보고한바있다. 본저자들의경우에서도하나의나사못으로고정한경우 5예에서나사못해리및불유합의소견이관찰되었다. Mullis 와 Sagi 16) 에의하면천장관절의강직은임상결과에어떠한영향도미치지못하며, 천장관절의해부학적정복과골유합이더좋은임상결과를보이기에천장관절의해리를동반한골반환손상환자에게는정확한해부학적정복이필요하다고강조하였다. 또한, 여러생역학적연구들에서 Tile C 형과같은불안정성골반환골절의경우후방고정시 2 point 고정이더많은안정성을준다는것이밝혀져있다 6,10,11,13). 그러나비록 2 point
불안정성골반골절의치료시경피적천장골나사못의수와위치에대한비교 5 Fig. 3. (A, B) A 39 year-old male patient who has Tile type C unstable pelvic fracture which were right side fracture of S2, sacroiliac joint disruption and symphysis pubis diastasis on x-ray and 3D CT. (C, D) Percutaneous IS screws fixation on S1 and S2 including interfragmentary screw fixation about S2 fracture under C-arm guidance. (E) Postoperative X-ray shows stable fixation with well reduction of the unstable pelvis. 고정이이상적이기는하나적절한나사못의위치를선정하기란용이하지않다. van den Bosch 등 27) 에의한사체실험에의하면제 1 천추에하나의나사못으로만고정한경우보다제 1 천추에두개혹은제 1 천추와제 2 천추에하나씩의나사못을고정한경우천장관절의전이가적었으며강도면에서도탁월하였고, 제 1 천추에두개혹은제 1 천추와제 2 천추에하나씩의나사못을고정한경우간에는유의한차이가없었다고하였으며, 하나의나사못으로고정한경우회전변형에더영향받기쉽다고보고하였다. 이로써추가적인나사못의삽입이회전변형과부하에대한실패율을낮추는데중요한역할을한다는것을추정할수있다 9,19,22). 천장관절에나사못을고정하는방법은 Matta와 Saucedo 의방법을따라장골후면을통해제 1 천추체혹은제 2 천추체로삽입하는방법으로두개의나사못이수렴하는방향으로삽입하는방법과평행하게삽입하는방법이있다 13,25). 제 2 천추에나사못을정확하게삽입하였다하더라도제 1 천추신경 근이나오는부위와워낙가깝게위치하기때문에정복의소실이생기면신경손상의위험이있을수있어골감소증이있는환자에게는제 2 천추에삽입을고려해봐야한다고보고하였다 15). 하지만본저자의경우기본적으로제 1천추에나사못으로강한고정을시행하고추가적인나사못고정시안전영역이좁은경우제 2 천추에삽입하였고, 일부의고령환자에게도이를시행할수있었다. 하지만제2 천추에나사못을삽입하는방법은신경손상의위험이 0.5% 에서 7.7% 까지높게보고될만큼신경손상의위험요소가높기때문에흔히시행되는시술은아니다 27). 저자들은일단제 1 천추체에천장관절을관통하는나사못의나선이위치하게하여강한고정을시행한후 2차로제 2 천추체를향하여같은술식으로나사못내고정을하였는데일단제 2 천추체에유도핀을삽입하였으며 C-arm 으로세심한관찰후 reamer로확공시조금이라도제 2 천추신경공을침범하는듯하면더이상의
6 김원유ㆍ지종훈ㆍ권오수외 3 인 reaming 은피하고나사못을삽입하였는데 12예중 3예는천골익부위까지만고정하기도하였다. Mowed 와 Geer 15) 에의하면제 1 천추의경우형태의변이가있는경우나사못의삽입이어려울수있고그러할경우제 2 천추에나사못을삽입하는방법이유용하다고하였고, 이에대한임상적안정성과효과에대해논한바있다. 본저자의경우에서도제 1 천추의안전영역이좁아두개의나사못을삽입하기어려운경우추가나사못을제 2 천추에삽입하였고신경손상의합병증은발생하지않았다. 제 2 천추의경우신경손상의위험이높기때문에술자의세심하고숙련된술기와충분한술전준비가필요하나본임상연구에서그결과는좋은것으로보아충분히가능한방법이라사료된다. 저자들의연구에서는일부의예에서동반손상이있는경우골반환의전방부의고정방법과이에대한고정력의차이가후방고정에미치는영향을고려하지않았기에부족한점이있으나이는추가적인비교분석이필요할것으로사료된다. 여러문헌의보고및본저자의경우와같이후방불안정성을가진골반환의골절에있어서는보다더안정적인내고정이필요하며경피적나사못을이용한내고정술을시행할경우두개이상의나사못고정으로내구성을높여주어야할것으로사료된다. 결론후방불안정성을가진골반환의손상에서제 1 천추에하나의나사못을사용한경우고정력이불충분할수있기에 1 천추나제 2 천추에추가적인나사못을이용해고정력을높이는것이좋을것으로여겨지며, 안전영역이좁은불안정골반환손상시에는제2 천추에서의추가삽입도권장될수있을것으로생각된다. 참고문헌 1. Bucholz RW: The pathological anatomy of Malgaine fracturedislocation of the pelvis. J Bone joint Surg Am, 63: 400-404, 1981. 2. Diliberti T, Reardon J, Lindsey R: The optimal radiographic intraoperative assessment in the placement of sacroiliac joint screws. Orthop Trans, 17: 1190, 1993-1994. 3. Dujardin FH, Roussignol X, Hossembaccus M, Thomine JM: Experimental study of the sacroiliac joint micromotion in pelvic disruption. J Orthop Trauma, 16: 99-103, 2002. 4. Ebraheim N, Rusin J, Coombs R, Jackson WT, Holiday B: Percutaneous computer tomography-stabilization of pelvic fracture: preliminary report. J Orthop Trauma, 1: 197-204, 1987. 5. Goldstein A, Phillips T, Selafani SJ, et al: Early open reduction and internal fixation of the disrupted pelvic ring. J Trauma, 26: 325-333. 1986. 6. Hoffmann E, Lenoir T, Morel E, Levassor N, Rillardon L, Guigui P: Posterior bridging osteosynthesis for traumatic sacroiliac joint dislocation: a report of seven cases. Eur J Orthop Surg Traumatol, 18: 47-53, 2008. 7. Keating JF, Werier MC, Blacht P, et al: Early fixation of the vertically unstable pelvis: the role of iliosacral screw fixation of the posterior lesion. J Orthop Trauma, 13: 107-113, 1999. 8. Kellam JF, McMurtry RY, Paley D, Tile M: The unstable pelvic fracture. Operative treatment. Orthop Clin North Am, 18: 25-41, 1987. 9. Kim JJ, Kim JW, Chang JS: Clinical outcome of AO type C pelvic ring injury. J Korean Orhtop Assoc, 40: 181-187, 2005. 10. Kim WY, Hearn TC, Seleem O, Mahalingam E, Stephen D, Tile M: Effect of pin location on stability of pelvic external fixation. Clin Orthop Relat Res, 361: 237-244, 1999. 11. Kim WY, Ji JH, Kim YY, Yang YJ, Lee DY: Anterior fixation techniques on unstable pelvic ring injury. J Korean Assoc, 40: 8-13, 2005. 12. Majeed SA: Grading the outcome of plevic fractures. J Bone Joint Surg Br, 71: 304-306, 1989. 13. Matta JM, Saucedo T: Internal fixation of pelvic ring fractures. Clin Orthop Relat Res, 242: 83-97, 1989. 14. McLaren AC, Rorabeck CH, Halpeeny J: Long-term pain and disability in relation to residual deformity after displaced pelvic ring fractures. Can J Surg, 33: 492-494, 1990. 15. Mowed BR, Geer BL: S2 Iliosacral screw fixation for disruption of the posterior pelvic ring: a report of 49 cases. J Orhtop Trauma, 20: 378-383, 2006. 16. Mullis BH, Sagi HC: Minimum 1-year follow-up for patients with vertical shear sacroiliac joint dislocations treated either
불안정성골반골절의치료시경피적천장골나사못의수와위치에대한비교 7 iliosacral screws: does joint ankylosis of anatomic reduction contribute to functional outcome? J Orthop Trauma, 22: 293-298, 2008. 17. Oliver CW, Twaddle B, Agel J, Routt ML Jr: Outcome after pelvic ring fractures: evaluation using the medical outcomes short from SF-36. Injury, 27: 635-641, 1996. 18. Pohlemann T, Gänsslen A, Schelwald O, Culemann U, Tscherne H: Outcome after pelvic ring injuries. Injury, 27(Suppl 2): B31-38, 1996. 19. Routt ML Jr, Kregor PJ, Simonian PT, Mayo KA: Early results of percutaneous Iliosacral screws placed with the patient in the supine position. J Orthop Trauma, 9: 207-214, 1995. 20. Routt ML Jr, Meier MC, Kregor PJ, Mayo KA: Percutaneous iliosacral screws with the patients supine technique. Oper Tech Orthop, 3: 35-45, 1993. 21. Routt ML Jr, Simonian PT, Mills WJ: Iliosacral screw fixation: early complications of the percutaneous technique. J Orthop Trauma, 11: 584-589, 1997. 22. Rubash HE, Brown TD, Nelson DD, Mears DC: Comparative mechanical performances of some new devices for fixation of unstable pelvic fractures. Med Biol Eng Comput, 21: 657-663, 1983. 23. Shuler T, Boone D, Gruen G, Peitzman A: Percutaneous iliosacral screw fixation: early treatment for unstable posterior pelvic ring disruptions. J Trauma, 38: 453-458, 1995. 24. Simpson LA, Waddell JP, Leighton PK, Kellam JF, Tile M: Anterior approach and stabilization of the disrupted sacroiliac joint. J Trauma, 27: 1332-1339, 1987. 25. Templeman D, Goulet J, Duwelius PJ, Olson S, Davidson M: Internal fixation of displaced fractures of the sacrum. Clin Orthop Relat Res, 329: 180-185, 1996. 26. Tile M: Pelvic ring fractures: should they be fixed? J Bone Joint Surg Br, 70: 1-12, 1988. 27. van den Bosch EW, van Zwienen CM, van Vugt AB: Fluoroscopic positioning of sacroiliac screws in 88 patients. J Trauma, 53: 44-48, 2002. = 국문초록 = 목적 : 불안정성골반골절에서천장관절의해부학적정복후 S1 혹은 S2 에천장골나사못을이용한고정술을시행한뒤나사못의위치와수에따른방사선학적및임상적결과를분석하였다. 대상및방법 : 천장관절골절및탈구를포함한 Tile 분류 C (AO/OTA) 의불안정성골반골절 31 예중 16 예에서는 S1 에한개의나사못을, 15 예에서는 S1 와 S2 에두개의나사못을이용한경피적고정술을시행하고, 평균 40.2 개월의추시관찰을하여방사선학적결과와 Majeed score 와 SF-36 을이용한임상적결과를분석하였다. 결과 : 수술후평균 40.2 개월 (12 76 개월 ) 관찰상 S1 에단한개의나사못으로고정한군중 5 예에서나사못의변위가있었으나두개의나사못으로고정한군에서는변위가관찰되지않았다. 경피적나사못고정술시좁은안전영역을보이는경우 S1 에두개의나사못으로고정하기어려운경우가있다. 본저자는좁은안전영역을가진불안정성골반환손상환자에게 S1 과 S2 에나사못을고정하여임상적으로좋은결과를보였다. 결론 : 불안정성골반환의정복과고정을위한두개의나사못을이용한경피적고정술은효과적이고믿을만한방법이며, 특히좁은안전영역을가진환자에게는 S1 에삽입후 S2 에추가로나사못의삽입이권장된다. 색인단어 : 불안정성골반골절, 경피적나사못고정술, 나사못의수와위치