knowledge of the anatomical variations of the axillary region has become more relevant with increasing surgeries of this region for breast cancer , reconstruction procedures and axillary by - pass .
lager 's arch or the axillary arch ( aa ) is the best - known variant structure in the axilla .
it is a muscular or fibromuscular slip of varying dimensions , extending from the latissimus dorsi ( ld ) muscle about the middle of the posterior axillary fold , and crosses over the neurovascular structures , to join the under surface of the tendon of the pectoralis major , the coracobrachialis , or the fascia over the bicepsbrachii [ 1 , 2 ] .
a 60-year - old female presented to our breast clinic with a left breast lump sized 3  2.5 cm incidentally felt since 3 months .
it was gradually increasing in size and was not associated with any other breast complaints .
she was evaluated with mammography that suggested a birads 3 lesion at 12 oclock position .
subsequently , her metastatic work - up included abdominal pelvic ultrasound , chest computed tomography and bone scan .
intra - operatively , during the axillary lymph node dissection , we encountered an unusual muscle slip crossing the axilla from the ld muscle to the posterior surface of the pectoralis major muscle anterior to the axillary vein .
all neurovascular structures and lymphoid tissue were lying posterior to this abnormal muscle ( fig .  1 ) . 

figure  1:aa muscle extending from ld muscle to under surface of pectoralis major tendon in the left axilla crossing over axillary neurovascular structures ( in view ) .
aa , axillary arch ; pm , pectoralis major ; ld , latissimus dorsi muscle ; av , axillary vein ; tnb , thoracodorsal neurovascular bundle ; ltn , long thoracic nerve . aa muscle extending from ld muscle to under surface of pectoralis major tendon in the left axilla crossing over axillary neurovascular structures ( in view ) .
aa , axillary arch ; pm , pectoralis major ; ld , latissimus dorsi muscle ; av , axillary vein ; tnb , thoracodorsal neurovascular bundle ; ltn , long thoracic nerve .
the lymph nodes lateral and beneath the arch were successfully dissected , and the arch itself was left undisturbed .
the procedure was uneventful , and the patient had a good post - operative recovery .
on follow - up , 21 months after surgery , the patient was alive and free of known disease .
however , it was karl langer in 1846 who gave a more accurate description of this variant , so thereafter , it was named after him [ 24 ] . throughout the literature , several terms have been used to describe the muscular variant running from the ld muscle towards the pectoralis major muscle : 
aa muscle ,  axillopectoral muscle and also their translations in different languages . in the following text ,
the anatomical description of the aa is variable among authors . according to testu 's classification ( 1884 ) , the complete aa extended between the ld and the tendon of the pectoralis major near its insertion on the humerus ; the incomplete one extended from the ld to the axillary fascia , biceps brachii muscle , coracobrachialis muscle , the distal end of the bicipital groove and the inferior edge of pectoralis minor muscle or coracoid process .
the case we encountered was of unilateral complete aa . according to many anatomic texts ,
however , it has been recognized in only 0.25% during axillary surgical procedures [ 1 , 2 , 4 , 5 ] .
the difference in surgical and anatomical incidence reflects a failure of reporting or identification during surgery .
also , this discrepancy may be attributed to the specific aim of cadaveric studies to identify such anatomic anomalies .
this should be kept in mind as it can be confused with axillary lymphadenopathy or soft tissue tumour [ 1 , 3 ] .
aa can act as entrapment site for the neurovascular bundle during some arm movements causing circulatory deficiency , chronic pain and paraesthesia .
simple division of the arch is curative in such cases [ 2 , 5 ] .
jelev , through his extensive work , introduced a new definition of  clinical aa as a variant muscular structure in the axilla that is a possible entrapment site for the nerves and vessels .
, the existence of a superficial or deep aa could be suspected according to the vessels or nerves mostly affected .
it can positively correlate its presence with neurovascular entrapment symptoms . also , it can assess its anatomic relations [ 1 , 5 ] .
the surgical significance of the aa is 2-fold : ( i ) it may hide some axillary nodes , and ( ii ) it may mislead the dissection into a supra axillary plane .
a group of lateral axillary nodes may be concealed under the aa while crossing over the axillary vein .
missing these nodes during axillary node dissection imposes a risk for local recurrence in patients with breast cancer and melanoma .
this also can lead to inaccurate staging , which in turn could negatively affect adjuvant and systemic therapy decisions for breast cancer .
this may lead the surgeon to dissect in a plane above the axillary vein increasing the risk of injury to the axillary artery and brachial plexus . during sentinel node biopsy ,
the aa can pose difficulty as it stretches in the hyper abducted position shifting the nodes higher . in order to clearly identify , the anatomic landmarks the arch can be divided at the level of the axillary vein .
furthermore , some authors suggest division of the arch in all cases to prevent possible post - operative axillary vein compression and associated lymphoedema [ 3 , 4 ] . in our case , there was no added morbidity related to dissection of nodes beneath and lateral to the arch .
the presence of the aa may precipitate lymphoedema in cases where ld myocutaneous flap is used for breast reconstruction .
for this reason , the aa should be divided if there is a possibility of a ld flap being required in the future . as in our case ,
most reported clinical cases describing the aa have been identified during axillary surgical procedures [ 2 , 4 ] .
it can cause confusion during routine axillary surgery for breast cancer , which can both affect procedure safety and misguide further treatment decisions [ 3 , 4 ] .
 langer 's arch is the best - known anatomic variant of definite surgical implication in the region of the axilla . 
 this rare anomaly is a muscular slip extending from the latissimus dorsi ( ld ) muscle to the tendons , muscles or fasciae around the superior part of the humerus . in this report 
 , we present a rare case of left axillary arch . during modified radical mastectomy for breast cancer , we encountered an abnormal muscle slip crossing the axilla from the ld muscle to the posterior surface of the pectoralis major muscle anterior to the neurovascular structures . 
 preoperative knowledge is essential to identify such unusual anomaly and avoid potential complications both intra- and postoperatively . 
INTRODUCTION
CASE REPORT
DISCUSSION
CONFLICT OF INTEREST STATEMENT