Millions of seniors rely on Medicare to cover their healthcare needs. And thankfully, the program covers a host of services, from hospital care to outpatient care and diagnostics.

But there are a number of important things Medicare will not pick up the tab for. Here are four you should know about.

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1. Dental services

Dental care is essential at any stage of life. But unfortunately, Medicare will not pay for general dental services that include cleanings, filling, and oral surgeries.

Now in some cases, Medicare will cover a dental exam or procedure if it relates to a covered medical procedure or situation. But routine dental care is not a Medicare-eligible expense.

If you want your dental treatments covered in retirement, you may want to consider signing up for a Medicare Advantage plan instead of sticking with original Medicare. Open enrollment for Medicare happens to be underway right now, so you have an opportunity to explore your Advantage plan options and see if there's a good fit for you.

Do keep in mind that Medicare Advantage has certain drawbacks, like limiting your provider network. And you won't always come out ahead financially with an Advantage plan. But it's still an option worth looking into.

2. Vision expenses

Many people experience vision loss or issues related to vision as they age. But unfortunately, Medicare will not pay for vision screenings or eyeglasses. If you want that service covered, you may need to sign up for a Medicare Advantage plan.

Now that said, Medicare generally will pay to treat an eye injury. And patients with medical conditions that put them at a higher risk for eye disease may be eligible for certain screenings under original Medicare. But for the most part, don't expect Medicare to cover the cost of getting a new prescription or glasses.

3. Hearing aids

Hearing is another one of those things that can deteriorate with age. Unfortunately, Medicare won't pay for hearing tests or hearing aids -- but Medicare Advantage often will.

In fact, some Advantage plans offer a host of supplemental benefits beyond what original Medicare will cover, so it does pay to explore those options while you can. You have until Dec. 7 to sign up for an Advantage plan if you're currently enrolled in original Medicare.

4. Long-term care

Medicare might pay for you to stay in a skilled nursing facility following an injury. But it won't pay for you to stay in an assisted living facility because you're losing mobility and need help with everyday functions like showering and dressing.

In fact, Medicare won't cover the cost of long-term care at all, whether it comes in the form of a home health aide or a nursing home. These services are generally considered custodial, not medical, and that's where Medicare draws the line.

Unfortunately, Medicare Advantage may not come to your rescue in the context of long-term care, as it generally won't pay for care that isn't medical in nature, either. For this reason, it's a good idea to consider putting long-term care insurance in place.

If you're already old enough to be a Medicare enrollee, you might struggle to secure affordable long-term care coverage. But if you're younger, you may find that you're able to qualify for insurance whose cost you can swing. Putting a policy in place might save you from catastrophic expenses down the line.

While Medicare offers a host of benefits to enrollees, clearly, there are some gaps to be filled. Consider a Medicare Advantage plan and long-term care insurance so you're not left scrambling.