INTRODUCTION

The psoas major muscle arises from T12 to the L5 vertebrae, extending caudally along the retro-peritoneum and proceeds downwards across pelvic brim and under the inguinal ligament and then anterior to the capsule of the hip joint. The psoas tendon is joined by the fibres of iliacus muscle in the pelvis and together they insert into the lesser trochanter of the femur. The ilio-psoas is the primary flexor of the hip joint and is also an important antigravity postural muscle, which helps humans to  maintain an erect posture. The close proximity of ilio-psoas to sacro-iliac and hip joints during its course predisposes these joints to possible spread of infection.

CASE REPORT

A 34-year-old senegalese female, previously fit and healthy was admitted with a 2- week  history of  fever,  pain,  and  swelling  of the  right  thigh  and  left  wrist. On admission,   she   was   pyrexial   and   unable   to   weight   bear   on  the   right   hip. Haematological and biochemical investigation revealed erythrocyte sedimentation rate
of 79, C-reactive protein of 319, and white cell count of 27.8x 109  with a neutrophilia.

A computerised tomography (CT) scan of the abdomen and pelvis revealed bilateral psoas abscesses tracking from the lumbosacral spine towards both hip joints (Fig 1).

Figure 1

Axial CT image showing large area of low attenuation collection in relation to left psoas (large white arrows) and small area of low attenuation collection in relation  to right psoas (small arrow)

The  patient  was  then  commenced  on  intravenous  benzyl  penicillin  and  surgical drainage of the  thigh and wrist abscesses were performed by the general surgical team.  A CT guided aspiration of the left psoas abscess was performed and 25ml of pus was   obtained   which   grew   streptococcus   pneumonia. Despite   multiple investigations no primary focus of infection could be found.  Investigation involved gynaecological, surgical and rheumatological teams. The patient was discharged home following a period of 6 weeks in hospital on intravenous antibiotics.

The patient presented to the orthopaedic team with a painful right hip and difficulty in walking four  months post discharge.   Examination of the hip revealed 30º of fixed flexion and globally restricted range of movement. There was 1 ½ cm of shortening of the left leg.  Movements of the left hip were pain free.  X-ray of the pelvis revealed there had been a disruption of the symphysis pubis and  sacro-iliac joint on the left resulting  in  a cephalad  migration of the  left  hemi pelvis.The  same  x-ray also revealed a destructive arthrosis of the right hip consistent with septic arthrosis (Fig 2). An  aspiration of the right hip did not grow any organisms in view of the fact this patient had been on a prolonged course of antibiotics.
The patient was investigated with an isotope bone scan that revealed the infection was quiescent.  An ESR and CRP confirmed no active infection.  Following a 3 months course  of  oral  penicillin   (with  advice  from  microbiologist),  she  underwent  an uncemented total hip replacement (Fig 3)  and made an uneventful recovery and is now walking pain free with equal leg lengths.

Figure 2

AP x-ray of pelvis showing loss of joint space with femoral head necrosis consistent with septic arthritis. Disruption of pubic symphysis and subluxation of sacro-iliac joint also seen

Figure 3

AP x-ray of pelvis showing uncemented right total hip replacement. Disruption of   pubic symphysis noted.

DISCUSSION

Pelvic ring  disruption without  an associated  trauma  is  a rare event .  Pelvic ring disruption due to psoas abscess infection is previously unreported. However, septic arthritis  of  the   hip  as  a  complication  of  psoas  abscess  information  is  well documented2. Mycobacterium  tuberculosis is a common pathogen in psoas abscess formation and is often secondary to spinal tuberculosis in developing countries; this is rarely the case in developed countries. Ricci et al. noted staphylococcus aureus was the  most  commonly  recovered  pathogen  in  those  with  primary  psoas  abscess3. Franco-Paredes   et   al.reported   a   case   of   psoas   abscess   co-infection   with staphylococcus aureus and mycobacterium tuberculosis4. In our patient the organism isolated was pneumococcus, which is rare and not reported in the literature.

The   treatment   of   a   pyogenic   abscess   has   traditionally   included   appropriate antimicrobial   therapy   and   surgical   drainage.   More   recently,   a   trend   toward percuteneous drainage has  become popular. Haaga reported 82-90 % success with percuteneous drainage in simple psoas abscess formation, but limitations such as siz e
and location of the abscess are important5. In our case 25ml of pus was aspirated from the left sacro-iliac joint, but there was no evidence to prove adequacy of drainage.

Despite negative aspiration from the right hip the presumed cause of destruction of this patient’s joint was one of infection.  As such, a total of 8 months was left between the primary infection of the  joint and replacement surgery.  At operation leg length was particularly difficult to achieve in view  of the significant shortening in the left contralateral leg secondary to hemi pelvic disruption.

The authors believe that aggressive surgical and antimicrobial treatment is needed during the hospital stay, and a high index of suspicion for complications and periodic follow up is necessary in patients with a psoas abscess. The consequences are grave if the complications are diagnosed late.