nerve / vascular injury , bone fracture , deep venous thrombosis , etc . are known as complications during total hip arthroplasty ( tha ) . concerning the installation of implants , it is common to implement trials of the femoral stem or head , and acetabular socket , before installing implants , in order to secure accuracy , and to check the degree of fixation , stress in the soft tissues , joint stability and impingement . in this paper , we describe our experience with cases in which the trial femoral head fell into the pelvis and migrated , during the intraoperative trial of the head . in the previous reports , seven cases were reported in the united states and europe , but there were only a few cases with the typical images of migrated trial head . this case is a first report in japan with a characteristic image . in this study the patients and their families were informed that the data from the cases would be submitted for publication , and all gave their consent . a 75-year - old female had been suffering from right hip pain at the age of 74 , and visited our hospital as an outpatient . bilateral osteoarthritis of the hip was found in a plane x - ray image of the hip joints ( figure 1 ) . because the right - side coxalgia gradually worsened , she was admitted to our hospital . she had a total of 55 points in the harris hip score before the surgery . the installation angle of the acetabular cup was set , in compliance with the pelvic guide , at 45 inclination , and 20 anteversion . after implementing acetabular reaming and an acetabular cup trial , an acetabular socket of 50 mm and a ceramic liner ( trident , stryker co. nj , usa ) were installed . subsequently , after implementing femoral reaming and rasping , a femoral stem component ( supersecurefit , stryker co. nj , usa ) was installed , as is typical . finally , the presence of stress in the soft tissues , joint instability and impingement were assessed , by using a trial femoral head of 26 mm . when an assistant , who did not have much experience in handling hip prosthesis , checked the anterior instability at external rotation in full extension , he forcibly caused an anterior dislocation . therefore , the trial femoral head fell anteriorly and migrated within the pelvis through the site which resected anterior joint capsule . not only was the retrieval unsuccessful , but the head was pushed even further inside with the fingers . the anterior portion of the joint capsule was opened up but we failed to find the trial head . we thought it was necessary to change the body position and requested an abdominal surgeon for help to retrieve the trial femoral head . it was necessary to operate on some day , if patient complaint abdominal pain or discomfort . upon obtaining the consent from the patient s family , the wound was closed to finish the surgery , leaving the trial femoral head inside . the x - ray image of the ap view just after the surgery showed that trial head was confirmed slightly ( figure 2 ) . after the surgery , with ct , the trial femoral head was confirmed inside the iliac muscle , within the quadrilateral space inside the pelvis ( figure 3-a , b ) . no clinical symptoms , such as abdominal symptoms , have been found during this period of time . the location of the trial femoral head has not changed during the past 3 years , according to the x - ray image ( figure 4 ) . furthermore , the current harris hip score has improved to a total of 81 points . narrowing of the cleft between articulations , spur formation and osteoscierosis in the acetabulum were observed , and a diagnosis of osteoarthritis was reached . ( coronal image ) the trial femoral head is observed to be inside the iliac muscle of the quadrilateral space within the pelvis . ( axial image ) the trial femoral head is found in the quadrilateral space in the posterior inner wall of the right pelvis . ( arrow head ) this shows an x - ray image 3 years after surgery . a 50-year - old female presented with advanced osteoarthritis of left hip secondary to acetabular dysplasia . the plane x - rays image showed grade4 by the kellgren - lawrence classification , and type1 by the crowe classification . before surgery the installation angle of the acetabular cup was set , in compliance with the pelvic guide , at 45 inclination , and 20 anteversion . after implementing acetabular reaming and an acetabular cup trial , an acetabular socket of 48 mm and a ceramic liner subsequently , after implementing femoral reaming and rasping , a femoral stem component ( supersecurefit , stryker co. nj , usa ) was installed , as is typical . finally , the presence of stress in the soft tissues , joint instability and impingement were assessed , by using a trial femoral head of 28 mm . when the assistant checked the anterior at external rotation in full extension , he forcibly caused anterior dislocation . at this time , the trial femoral head fell anteriorly and migrated within the pelvis through the site which resected anterior joint capsule . because we had experienced case 1 , we did not try to retrieve it using fingers . the anterior wall of the pelvis was blocked by pressing from the surface of the skin around the groin region , and a large size kocher was inserted in the anterior of the trial femoral head , to block any further migration in a deeper area . it was retrieved manually . when retrieving , no nerve / vascular injuries were observed . no complications were observed after 8 months of surgery , and the patient is progressing favorably . reports associated with the fall/ migration of a trial femoral head into the pelvis , as a complication during tha , are in existence for 7 cases , with a total of 11 examples so far . however , it is believed that the frequency of occurrence is minimal . with respect to the surgical approach on the occurrence of the fall/ migration of trial femoral head into the pelvis , the hardinge approach , anterior approach and posterior approach have been reported . in other words with respect to the occurrence situation , a slight tightness in the soft tissues was observed and no instability was mentioned . therefore , it does not mean that a gentle stress in the hip joints or instability tends to cause the problem , but rather , it is assumed that it tends to occur when excessive tightness is observed . in these case , when posterior impingement or anterior instability is assessed with external rotation in full extension , migration occurred because the assistant forcibly caused anterior dislocation . it was believed that the trial femoral head was flipped and migrated deeper in the pelvis , because of the additional excessive stress on the hip joint . in addition , the possibility was that it was easy to be dislocated forward because the anteversion of the acetabular cup of case1 was moderately excessive . batouk at al . reported that the region in which the trial femoral head fell and migrated was the anterior of the sacroiliac joint , while alfonso et al . reported it was inside the retroperitoneum of the exterior of ilioposoas muscle , callaghan et al . reported that it was the inside surface of the psoas tendon and wes et al the trial femoral head was confirmed to be in the quadrilateral space within the pelvis , in an area lower than previous reports . it is believed that this was because the trial femoral head was flicked into the lower area , due to the dislocation in a high extension position , and furthermore because the surgeon pushed further with his fingers .   according to the previous report , there were only 3 examples in which trial femoral heads were left behind without being retrieved , while they were retrieved in 8 samples . with respect to the retrieval method , it has been reported that aortic cross - clamping was used for retrieval , or an ilio - inguinal approach was developed for retrieval , or when it migrated to the retroperitoneum , it was retrieved under laparoscopic vision . how to handle retrieval of a trial femoral head is not difficult if it is retrieved in a calm manner . however , in case 1 , because the surgeon tried to retrieve it with fingers from the very beginning , he was unable to grasp it properly and it slipped , resulting in further dislocation into a deep area of the pelvis . retrieval from the original surgical wound was not possible . because the corrosion of the trial head and the further movement were possible , it was sure that the retrieval was desirable . however , upon obtaining informed consent from the patient and her family , further efforts at retrieval were not attempted . it is recommended that a retrieval operation from the anterior be implemented together with an abdominal surgeon and vascular surgeon .   with regard to case 2 , retrieval was possible by blocking the anterior , based on the experience in case 1 . as shown in the above , when a trial femoral head is migrated into the pelvis , the retrieval becomes difficult . when we enlarge the surgical site , we must resect anterior joint capsule to obtain the better visual field and to release the hip contracture . if anterior joint capsule was completely resected , a trial head will easily migrate to the anterior space under the rectus femoris muscle ( figure5 ) . because we recommend that the anterior joint capsule resection should be a minimum . the extension and abduction test of the hip carried out because we should confirm anterior instability and posterior impingement . we suggest to have put gauze through the neck to control a stem because dislocation with the force prevented . we should operate it to avoid nerves , vascular injury if we perform grasp of a trial head with forceps as far as we can look it in the face . it is recommended , as a means of preventing migration , that an anterior block be implemented with gauze packing , if stress in the joint is strong , or a " necklace " be used , threading the trial femoral head . in conclusion we have experienced 2 cases in which migration of the trial femoral head into the pelvis during tha was observed , both of which were intraoperative complications . if the trial femoral head migrated , retrieve the trial head in a calm manner so that it will not further migrate into a deep area . when anterior joint capsule was resected , the trial head arrived at the pelvic medial wall through the inside of the rectus femoris muscle .
abstractvarious complications occur during total hip arthroplasty ( tha ) . it is common to implement trial treatments of various implants during tha . here 
 we report on 2 cases in which the trial femoral head fell into the pelvis and migrated during tha . 
 the trial head was not retrieved but left inside in one case , while the other presented the opportunity for a possible retrieval . 
 it is important to recognize the existence of such rare complications .