The multibillion-dollar spine industry faces mounting demands to prove the value of back surgery. In recent years, spine sales have surged for companies such as Medtronic ( MDT) and Zimmer ( ZMH). The companies have won over surgeons and patients alike with new technologies that promise welcome relief from severe back pain. But Medicare is now seeking hard, scientific evidence that can support the huge jump in fusion surgeries among seniors and other patients that it covers. Prior to a big meeting in late November, Medicare hinted that past studies -- already lacking in number and scope -- fall short of justifying such operations. Then, just a week before the Medicare panel convened, results from the biggest spine study of its kind rolled in. On the surface, at least, that study -- popularly known as Sport -- suggested that patients fare equally well whether they undergo back surgery or not. But prominent spine experts, including consultants for Medtronic, showed up to attack the study and caution Medicare officials against jumping to conclusions. Spine surgeons worried that Medicare might confuse operations for two very different back problems -- on the basis of a study they consider flawed -- and stir up reimbursement nightmares in the process. Ultimately, Medicare panelists listened to those experts. The agency will now wait on new studies before revisiting its coverage of spinal fusions.