PMT String Back LSO Back Brace - XXXXL by PMT MEDICAL. $125.00. The String Back is a rigid LSO back brace with a very snug fitting posterior rigid panel and a comfortable anterior rigid panel. The string back technology allows for a zip-sinch type of suction allowing the lower back to decompress, and ultimate relieve pain and provide support. Available in Small,Medium,Large,XL,XXL,XXXL and XXXXL sizes.