Many of the original Pioneer ACO's are no longer operating.
Accountable Care Organizations are perhaps a synonym for Health Maintenance Organization. Without going into detail here, I refer the reader to the links in this sentence. The HMO offers a fixed payment for services to providers by means of a contract.
Types of HMOs
Although businesses pursued the HMO model for its alleged cost containment benefits, some research indicates that private HMO plans don't achieve any significant cost savings over non-HMO plans. Although out-of-pocket costs are reduced for consumers, controlling for other factors, the plans don't affect total expenditures and payments by insurers. A possible reason for this failure is that consumers might increase utilization in response to less cost sharing under HMOs. Somehave asserted that HMOs (especially those run for profit) actually increase administrative costs and tend to cherry-pickhealthier patients.HMOs (especially those run for profit) actually increase administrative costs and tend to cherry-pick healthier patients.
ACO Payment Models
In an effort to lower healthcare costs, the CMS has introduced the one-sided and two-sided payment model, either of which the ACOs can choose to adopt. Under the March 2011 proposal, ACOs could choose the one-sided model, in which they would participate in shared savings for the first two years and assume shared losses in addition to the shared savings for the third year. In the two-sided model, ACOs would participate in both shared savings and losses for all three years. Although the ACO assumes less financial risk in the one-sided model compared to the two-sided model, ACOs have a maximum sharing rate of 50% in the one-sided model and a higher maximum sharing rate of 60% in the two-sided model, provided that the minimum shared savings rate threshold of 2% is reached. For both models, there is a shared loss cap that increases each year. However, initial feedback raised concerns regarding ACO's financial risk and possible cost savings. On October 20, 2011, DHHS released the final regulations that altered providers' financial incentives. Under the one-sided model, providers no longer assumed any financial risk throughout the three years and continued to have the opportunity to share in cost savings above 2%. Under the two-sided model, providers will assume some financial risk but will be able to share in any savings that occur (no 2% benchmark before provider savings accrue).
In addition to these cost/risk features, ACO Quality Measures are clearly defined in ACO regulations and operations.
To address the goal of improving healthcare quality, CMS has established five domains in which to evaluate the quality of an ACO's performance. The five domains are "patient/caregiver experience, care coordination, patient safety, preventative health, and at-risk population/frail elderly health." 
In theory the ACO engages the following participants:
As networks of providers, ACOs are composed mostly of hospitals, physicians, and other healthcare professionals. Depending on the level of integration and size of an ACO, providers may also include health departments, social security departments, safety net clinics, and home care services. The various providers within an ACO will need to work with one another to provide coordinated care to the beneficiary population, align incentives and lower overall healthcare costs. Although ACOs have been compared to health maintenance organizations (HMOs), ACOs are different in that they allow providers much freedom in developing the ACO infrastructure. Any provider or provider organization may assume the leadership role of running an ACO, as the ACA does not explicitly designate any provider to that role.Payers
The federal government, in the form of Medicare, will be the primary payer of an ACO. Other payers include private insurances, or employer-purchased insurance. Payers may play several roles in helping ACOs achieve higher quality care and lower expenditures. Payers may collaborate with one another to align incentives for ACOs and create financial incentives for providers to improve healthcare quality.
An ACO's patient population will primarily consist of Medicare beneficiaries. In larger and more integrated ACOs, the patient population may also include those who are homeless and and uninsured. Patients may play a role in the healthcare they receive from their ACOs by participating in their ACO's decision-making processes.their ACO's decision-making processes.
CMS Is the Reason We Have so Little Useful ACO Research | THCB