Aetna Inc said it was able to hold down costs by directing more of its plans to US medical providers that focus on efficient care. Aetna said that next year it will shift even more of its commercial and Medicare business to groups known as accountable care organizations, which are offered incentives to improve the quality of care while lowering costs. The shift should help it weather government efforts to rein in reimbursement for Medicare and the Medicaid program for the poor, as well as expected fees and taxes on the industry down the road.